Sonar: Added analog sonar support

[ghost]

Added sonar support using the AP_Rangfinder_analog class instead of the
Maxbotix class. This brings to same as arduplane and ardurover. This
class is much nicer because we can tweek sonar values through GCS
parameters. Downside is now sonar selection is not trivial, may need to
implement simple config on GCS side or add a wiki for default settings
of most common analog sonars.
-Added enabled(bool) setter function to AP_Rangefinder_analog in order
to dynamically enable and disable sonar with chan7 or chan 8 options
-Added check to sonar health if sonar_alt - baro_alt < 10 meters to
prevent fly aways if sonar failed, or is covered up. **Not tested, needs
testing
-Added sonar_alt and sonar voltage reports to mavlink GCS, now displays
in mission planner correctly.
-User should set max_distance in parameters to appx 60% of max for
reliability reasons, because built in 60% check had to be removed.
-Added voltage report to CLI Test Sonar
-added option in apm_config.h to enable sonar tilt correction if one so
chooses to use this feature.

*Tested on Pixhawk, sonar pin 15 maxbotix MB1240XL, scale factor 2.42
*I don't this will fit on APM anymore?

[rmackay9]

Supergoldfish,
I didn't see your pull request but this is definitely something we want to do. I might grab what you've done here and pull it in anyway.

[ghost]

OK Randy. I closed it because I didn't like the way I did the sonar health check against baro alt. I was attempting to prevent fly-aways if the copter sonar alt to baro alt was greater than 10 meters, (In the case of blocked sonar), but that solution was problematic because I didn't account for un-even terrain since baro alt is based on Home..

I can re-submit minus that check if you like, or remove that check on integration.

[rmackay9]

David,
Ok, I didn't see that check so I'll remove that on intetgration. I think the solution for the sonar health is going to be to encourage people to move to the I2C sonar instead. They're the same price and it's a more sophisticated interface.
Txs again.