when MANUAL_LEVEL is set to 1, we don't do accelerometer levelling at
startup, and instead used the values saved in the EEPROM. This makes
it easier to do levelling on the bench, or once for a series of
flights for the day

when a user first connects with USB, and later switches to the
telemetry port without restarting we were getting zero for error_yaw
in the logs, as AHRS.get_error_yaw() was being called twice.

This ensures we give the last value after the counter is reset

explain the algorithm a bit more

        #define CH6_YAW_KI 24
        #define CH6_ACRO_KP 25
        #define CH6_YAW_RATE_KD 26
        #define CH6_LOITER_KI 27
        #define CH6_LOITER_RATE_KI 28

AP_OpticalFlow - small bug fix to ensure init attempts to read the product id 3 times before giving up

add read delay on write for serial
add tlog to gpx creation
fix issue 583 - georef image with a tlog

this caused problems with random()

This adds a large amount of noise robustness to the compass offset
learning algorithm, at a cost of 120 bytes of memory. The changes are
based on a long discussion with Bill Premerlani.

we don't need to add count any more, as floating point maths doesn't
need to round up

this seems to work much better than the old algorithm, converging
faster and more accurately. Even better, it has no linkage to DCM, so
no possibility of nasty feedback effects

fix antenna tracker issue with saved settings
add pretyping to config list
add rawlogs
modify mavlink log graph interface. 2 x yaxis & roll,pitch,yaw now in degs
update dataflash log format - thanks randy
update google earth interface refresh to 0.3 sec

APM_RC - moved Force_Out0_Out1, Force_Out2_Out3 and Force_Out6_Out6 to APM_RC parent class because it's already implemented in the APM1 and APM2 child classes anyway

Filter - added simple LowPassFilter (simple but it's possible to make errors with simple stuff too so might as well have one)

AP_OpticalFlow - resolved compile error in example sketch (it could not find DCM.h but it's not required anyway)

ArduCopter - updated to firmware version to 2.5.2 to avoid confusion with post 2.5.1 versions posted to downloads area

add ardutracker support
add load/save setting in Tracker
move getserialports to serial class
hopefully fix dtr issue for good.

The simulator flies very badly when using the full acceleration
code. I don't yet know if this is a bug in the simulation or a problem
with AHRS not handling acceleration for multicopters.

For now set the acceleration to be half a 'pure gravity' acceleration
and half from the full 3D calculation.

this is equivalent to multiplying by m.transposed(), but is more
efficient