Copter: tradheli's implement spool logic

[rmackay9]

This PR is a subset of the contents of this PR from bnsgeyer. The main purpose of this PR is to make TradHeli's "spool logic" interface and implementation more consistent with Multicopters. We hope this will reduce the number of tradheli specific exceptions in the main code and make it easier to keep TradHeli up-to-date with new Multicopter features.

To provide some background, both TradHeli's and multicopters already implement "spooling" which means gradually increasing or decreasing the maximum turn rate of the motors but the implementation is very different. Even after this PR is merged, the parameters holding the different maximums are different but it is still a step in the right direction.

Multicopter parameters / TradHeli parameters:

• MOT_SPIN_MIN, MOT_SPIN_MAX / No equivalent
• MOT_SPIN_ARM / H_RSC_IDLE
• MOT_SPOOL_TIME / H_RSC_RAMP_TIME & H_RSC_RUNUP_TIME

The major changes this makes to master are:

• AP_Motors library's desired and actual spool state variables are moved from the Multicopter class up to the parent class (to allow TradHeli's to use them). Also the SPIN_WHEN_ARMED state of each is renamed to GROUND_IDLE.
• AP_MotorsHeli implements the spool logic. Previously tradheli's completely ignored the vehicle code call's to set_desired_spool_state()
• Copter vehicle code changes to reduce functional changes to Heli. TradHelis landed in non-autonomous modes (PosHold, Loiter, Drift, ZigZag, etc) only spool down the main rotor once the user disengages the interlock. In Multicopters they spool down once landed.
• Copter, Sub, Plane and AC_AutoTune updated to use renamed GROUND_IDLE spool state

The TradHeli changes have been extensively tested by @bnsgeyer in RealFlight. There should be no functional changes to multicopters as a result of this PR.

As a side note there will be a follow up PR which makes further changes to the Copter vehicle code to make better use of the actual spool state. This including delaying the disarm until the main rotor has spooled down.

[rmackay9]

Merged after testing with vagrant as discussed on the dev call