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
Plane: fixed flash logging of ATT 

fixed types to be int16
AP_Rangefinder: added units for settle time
HAL_PX4: make PX4 snprintf support %S format

just maps it to %s for normal printf
SITL: support vsnprintf with %S on SITL
AP_InsertialSensor: added support for MAVLink user interaction

allows APM to ask user to print enter to continue via MAVLink messages
during accel calibration
AP_InertialSensor: show calibration offsets for success and failure

Pair-Programmed-With: Randy Mackay <rmackay9@yahoo.com>
Copter: use new accel cal interact over MAVLink
Plane: support 3D accel calibration over MAVLink
AC_Fence: fixed example build
AC_WPNav: fixed example build
AP_Baro: fixed example build
AP_InertialNav: fixed example build
AP_InserialSensor: fixed example build