AP_Motors: accessors to set roll, pitch, yaw, throttle

This saves 16bytes of RAM and slightly reduces the dependence upon the
RC_Channel class as the interface.
AP_MotorsMatrix: use ref for roll, pitch, yaw, thr channels
TradHeli: use refs for all RC_Channels
SingleCopter: use refs for all RC_Channels
TriCopter: use refs for all RC_Channels
AC_AttControl: use motor accessors to set roll, pitch, yaw, thr

Saves 8bytes of RAM
AC_AttControlHeli: use motor accessors to set roll, pitch, yaw, thr
Copter: integrate refs for RC_Channels in motor defs