SRV_Channel: convert function to AP_Int16

[IamPete1]

Fixes #12744

This is changes SERVOx_FUNCTION from AP_INT8 to AP_INT16. The param conversion does not work, I need some advice on how to do it properly.

[magicrub]

@peterbarker might know about the conversion. Otherwise LGTM

[peterbarker]

Actually, @rmackay9 knows how these conversion table thingies work.

The comment in Rover's Parameters.cpp looks helpful.

[andyp1per]

https://ardupilot.org/dev/docs/code-overview-adding-a-new-parameter.html#changing-the-type-of-a-parameter

[IamPete1]

I have got all the keys, now going to try and work out the conversion

Plane:
Key 150: Index 0: GroupElement 20550 : SERVO1_FUNCTION: 77
Key 150: Index 0: GroupElement 20614 : SERVO2_FUNCTION: 78
Key 150: Index 0: GroupElement 20678 : SERVO3_FUNCTION: 6
Key 150: Index 0: GroupElement 20742 : SERVO4_FUNCTION: 7
Key 150: Index 0: GroupElement 20806 : SERVO5_FUNCTION: 34
Key 150: Index 0: GroupElement 20870 : SERVO6_FUNCTION: 33
Key 150: Index 0: GroupElement 20934 : SERVO7_FUNCTION: 35
Key 150: Index 0: GroupElement 20998 : SERVO8_FUNCTION: 36
Key 150: Index 0: GroupElement 21062 : SERVO9_FUNCTION: 0
Key 150: Index 0: GroupElement 21126 : SERVO10_FUNCTION: 0
Key 150: Index 0: GroupElement 21190 : SERVO11_FUNCTION: 0
Key 150: Index 0: GroupElement 21254 : SERVO12_FUNCTION: 0
Key 150: Index 0: GroupElement 21318 : SERVO13_FUNCTION: 0
Key 150: Index 0: GroupElement 21382 : SERVO14_FUNCTION: 0
Key 150: Index 0: GroupElement 21446 : SERVO15_FUNCTION: 0
Key 150: Index 0: GroupElement 21510 : SERVO16_FUNCTION: 0

Copter:
Key 99: Index 0: GroupElement 20560 : SERVO1_FUNCTION: 33
Key 99: Index 0: GroupElement 20624 : SERVO2_FUNCTION: 34
Key 99: Index 0: GroupElement 20688 : SERVO3_FUNCTION: 35
Key 99: Index 0: GroupElement 20752 : SERVO4_FUNCTION: 36
Key 99: Index 0: GroupElement 20816 : SERVO5_FUNCTION: 0
Key 99: Index 0: GroupElement 20880 : SERVO6_FUNCTION: 0
Key 99: Index 0: GroupElement 20944 : SERVO7_FUNCTION: 0
Key 99: Index 0: GroupElement 21008 : SERVO8_FUNCTION: 0
Key 99: Index 0: GroupElement 21072 : SERVO9_FUNCTION: 0
Key 99: Index 0: GroupElement 21136 : SERVO10_FUNCTION: 0
Key 99: Index 0: GroupElement 21200 : SERVO11_FUNCTION: 0
Key 99: Index 0: GroupElement 21264 : SERVO12_FUNCTION: 0
Key 99: Index 0: GroupElement 21328 : SERVO13_FUNCTION: 0
Key 99: Index 0: GroupElement 21392 : SERVO14_FUNCTION: 0
Key 99: Index 0: GroupElement 21456 : SERVO15_FUNCTION: 0
Key 99: Index 0: GroupElement 21520 : SERVO16_FUNCTION: 0

Rover:
Key 58: Index 0: GroupElement 20547 : SERVO1_FUNCTION: 26
Key 58: Index 0: GroupElement 20611 : SERVO2_FUNCTION: 0
Key 58: Index 0: GroupElement 20675 : SERVO3_FUNCTION: 70
Key 58: Index 0: GroupElement 20739 : SERVO4_FUNCTION: 0
Key 58: Index 0: GroupElement 20803 : SERVO5_FUNCTION: 0
Key 58: Index 0: GroupElement 20867 : SERVO6_FUNCTION: 0
Key 58: Index 0: GroupElement 20931 : SERVO7_FUNCTION: 0
Key 58: Index 0: GroupElement 20995 : SERVO8_FUNCTION: 0
Key 58: Index 0: GroupElement 21059 : SERVO9_FUNCTION: 0
Key 58: Index 0: GroupElement 21123 : SERVO10_FUNCTION: 0
Key 58: Index 0: GroupElement 21187 : SERVO11_FUNCTION: 0
Key 58: Index 0: GroupElement 21251 : SERVO12_FUNCTION: 0
Key 58: Index 0: GroupElement 21315 : SERVO13_FUNCTION: 0
Key 58: Index 0: GroupElement 21379 : SERVO14_FUNCTION: 0
Key 58: Index 0: GroupElement 21443 : SERVO15_FUNCTION: 0
Key 58: Index 0: GroupElement 21507 : SERVO16_FUNCTION: 0

Tracker:
Key 39: Index 0: GroupElement 321 : SERVO1_FUNCTION: 71
Key 39: Index 0: GroupElement 322 : SERVO2_FUNCTION: 72
Key 39: Index 0: GroupElement 323 : SERVO3_FUNCTION: 0
Key 39: Index 0: GroupElement 324 : SERVO4_FUNCTION: 0
Key 39: Index 0: GroupElement 325 : SERVO5_FUNCTION: 0
Key 39: Index 0: GroupElement 326 : SERVO6_FUNCTION: 0
Key 39: Index 0: GroupElement 327 : SERVO7_FUNCTION: 0
Key 39: Index 0: GroupElement 328 : SERVO8_FUNCTION: 0
Key 39: Index 0: GroupElement 329 : SERVO9_FUNCTION: 0
Key 39: Index 0: GroupElement 330 : SERVO10_FUNCTION: 0
Key 39: Index 0: GroupElement 331 : SERVO11_FUNCTION: 0
Key 39: Index 0: GroupElement 332 : SERVO12_FUNCTION: 0
Key 39: Index 0: GroupElement 333 : SERVO13_FUNCTION: 0
Key 39: Index 0: GroupElement 334 : SERVO14_FUNCTION: 0
Key 39: Index 0: GroupElement 335 : SERVO15_FUNCTION: 0
Key 39: Index 0: GroupElement 336 : SERVO16_FUNCTION: 0

Sub:
Key 88: Index 0: GroupElement 20560 : SERVO1_FUNCTION: 33
Key 88: Index 0: GroupElement 20624 : SERVO2_FUNCTION: 34
Key 88: Index 0: GroupElement 20688 : SERVO3_FUNCTION: 35
Key 88: Index 0: GroupElement 20752 : SERVO4_FUNCTION: 36
Key 88: Index 0: GroupElement 20816 : SERVO5_FUNCTION: 37
Key 88: Index 0: GroupElement 20880 : SERVO6_FUNCTION: 38
Key 88: Index 0: GroupElement 20944 : SERVO7_FUNCTION: 0
Key 88: Index 0: GroupElement 21008 : SERVO8_FUNCTION: 0
Key 88: Index 0: GroupElement 21072 : SERVO9_FUNCTION: 0
Key 88: Index 0: GroupElement 21136 : SERVO10_FUNCTION: 0
Key 88: Index 0: GroupElement 21200 : SERVO11_FUNCTION: 0
Key 88: Index 0: GroupElement 21264 : SERVO12_FUNCTION: 0
Key 88: Index 0: GroupElement 21328 : SERVO13_FUNCTION: 0
Key 88: Index 0: GroupElement 21392 : SERVO14_FUNCTION: 0
Key 88: Index 0: GroupElement 21456 : SERVO15_FUNCTION: 0
Key 88: Index 0: GroupElement 21520 : SERVO16_FUNCTION: 0

[IamPete1]

I have added a attempt at a param conversion table based on the above keys, its does not seem to work, so I must have missed something somewhere

[rmackay9]

The decision from the dev call was that @tridge would look into whether we need to change the parameter index or not. If we don't need to then we can avoid the need for parameter conversion code.

[tridge]

I have converted this PR to new a new AP_Param::convert_parameter_width() function that simplifies things a lot, and will allow us to easily widen parameters in future without using a new parameter key.
Note! This removes the old parameter conversion for RC* to SERVO*, which was used to convert from Copter 3.4