Adding new AC_Gimbal lib

minimal support for Brushless gimbals
- includes Tilt support
- manual control via RC aux channels
- ROI support
- compatible settings with AP_mount
- will grow to include Yaw support in future
AC_Gimbal import and support

replaces AP_Mount
control_auto

Adds support for AC_Gimbal. ROI's now handles as vectors rather than Lat/Lon location for more compatibility and accuracy.
Adding ROI pointing to Loiter

Allows user to toggle ROI mode in Loiter with Aux switch.
ROI in Loiter takes over Yaw, but leaves all other axis for user.
ROI triggering

Generates new ROI based on gimbal attitude. Assumes altitude of zero.
Remove AP_mount2
AC_Gimbal group param update
Throttle_assist upgrade

- added param for gain value
- added check for Inav v Baro divergence
- added Aux switch for enabling
Setup CLI

Added back the ability to set params from CLI