SITL: fixed compass vector to be based on yaw not GPS heading

Many thanks to Randy for noticing this, and apologies for the wasted time!
SITL: fixed drift rate limit in simulator
AutoTest: remove lower loiter PID overrides

SITL issues resolved now so default PIDs ok again
AP_InertialNav: use standard LATLON_TO_CM, DEG_TO_RAD constants
Copter: resolve compiler warning re fence
AP_AHRS: added AHRS_GPS_MINSATS option

if the number of visible satellites is below AHRS_GPS_MINSATS then
don't use the GPS for acceleration correction for attitude
AHRS: perf improvement by caching declination vector
AP_Math: added mulXY() for matrix3 and vector3

returns the XY components of the product
AP_Math: implement vector2 cross product

useful in DCM for faster yaw correction
AP_AHRS: optimise yaw drift correction

use new vector2 cross product, and factor out the z component of a dcm
mul_transpose() to reduce the number of floating point operations for
a yaw drift correction cycle
Copter: remove unused radius_of_earth constant
AP_Math: use DEG_TO_RAD in longitude_scale

Also increased accuracy of RadiansToCentiDegrees although it is like the
compiler will throw away the extra digits anyway.
InertialNav: use AP_Math's longitude_scale
Copter: disable centrifugal force correction when disarmed
AP_L1_Control: Cleaned up calculation of damping to reduce computations

also removed _cross2D, using math library instead
APM_Control: Added tuning guide readme
APM_Control : Removed 20Hz low-pass filters on rate gyro inputs.

These are not required due to MPU 6000 filtering.
APM_Control: APM tuning guide.txt - Added step by step tuning description.
APM_Control: APM tuning guide.txt - Minor editorial corrections
APM_Control: Added alternate tuning method for roll and pitch loops

The alternate tuning method for roll and pitch will give a better
results, but comes with more risk as it will result in a larger rate
feedback gain