TradHeli: ramp up changes

Rewrote tail and main rotor ramp up methods
Moved direct drive ESC speed control into rsc_control method
Pass in ch7 servo as servo_aux to TradHeli motors object constructor
split CH7_SETPOINT parameter into GYR_GAIN and DIRECTDRIVE parameters
replaced RSC_RATE with uint8_t RSC_RAMP_TIME parameter
rename GOV_SETPOINT parameter to RSC_SETPOINT
RSC_MODE parameter description updated to indicate it controls the
source of main rotor speed