SITL: added SIM_GPS_NUMSATS parameter

this allows testing of the DCM code with less than 6 satellites
AP_AHRS: fixed position estimate with 2D fix or low satellite count

we should still use the GPS for position fixes when we have a low
number of satellites, but we should stop using it for velocity
estimates and attitude correction.
APM_Control: Corrected typographical error in APM_Control/TuningGuide.txt
Plane: Added  a constraint to demanded bank angle during FBW-A mode to prevent roll limits being exceeded
AC_WPNav: remove unnecessary speed constraint
Rover: prepare for 2.41 release
WPNav: flexible loiter speed

Also includes bug fix to allow speeds > 10m/s during missions