Copter: Leonard's improved Loiter
Copter: fix barometer cli test
Copter: add baro health to pre-arm checks
Copter: resolve compiler warning re unused vars
Copter: initialise target vel in loiter
Plane: added MIXING_GAIN parameter

this allows for the full range of output on both channels, but can
saturate
Plane: use a default mixing gain of 0.5

this ensures compatibility with existing configs
Merge remote-tracking branch 'diydrones/master'