mttr-4.1 development branch

[tridge]

This issue describes the status of the 4.1 development branch, mttr-4.1
https://github.com/matternet/ardupilot/tree/mttr-4.1

key missing features in mttr-4.1:

• Support mission pause via DO_CHANGE_SPEED (Support mission pause via DO_CHANGE_SPEED #200)
• Implement change alt command in mission
• pre-arm checks based on GPS minsats and movement
• SNR logging for rangefinders
• custom magz EKF handling
• rangefinder complementary filter
• prevent descent while centering for precision landing
• configurable GPS difference and min sats for arming (should be lua)
• configurable parameters for mag limits (should be lua)

there have been significant parameter changes changes, with some index changes which will impact update of existing vehicles:

• batt type analog table changed to 21
• max navigation alt WP_NAVALT_MAX removed
• TOFF_BARO_DIP and TOFF_HOV_PCT removed

Some features have had significant code changes which warrant additional testing focus:

• precision landing has changed considerably, likely will need some code from 4.2 backported
• throttle mix changes during land descent
• continue after land mission option
• github actions for CI
• USER mission item for failsafe zone jumps
• removed WP_XTRACK_MAX, should not be needed
• need to test M2 json model
• RTCMv3 injection to UAVCAN GPS