An hidden bit of failsafe functionality in the set_mode function was
switching the throttle mode back to THROTTLE_AUTO instead of
THROTTLE_LAND

new set_accel_throttle_I_from_pilot_throttle function copies the
difference between the pilot's throttle and hover throttle to the
accelerometer based throttle controller's I term.

This effectively means that an RTL kicked off by a failsafe will
immediately switch to LAND mode only if within 2 meters.  Previously the
radius was much wider (15m).

This message takes 1ms to write and we are running over our limits on
some loops.

Jonathan Challinger added

this ensures the rate controllers use the latest data

this avoids problems in the scheduler turning up as errors in the DCM
calculation. The MPU6k produces samples with more reliable timing than
our scheduler does!

this saves a fair bit of CPU for ArduCopter

this saves 256 bytes of memory

this saves 32 bytes of memory

AP_Baro_BMP085: increase ratio of pressure reads to temperature reads to be 5:1 which is same as APM2's baro

previously the ratio was 1:1

Improved surface tracking by using slewed althold controller
Reduced sonar mode filter to just 3 elements to reduce lag but at the possible consequence of allowing sonar noise to creep through for people with margin sonar set-ups.

this fixes a problem with phantom GPS detection, which caused us to
receive the config bytes we send to the GPS on bootup back as valid
packets. This caused us to detect a phantom GPS.

this message is huge and slow, and is pointless as we have
num_satellites in GPS_RAW_INT

ArduCopter: save 10bytes by replacing multiple static instances of log_counter with a single global variable called pid_log_counter

pid_log_counter is used to limit the rate at which we log PID controller values

TradHeli: Fixing bug that causes Alt_Hold mode to use the Stab_Throttle scaling.  It should not have this scaling, it should have access to full swashplate range.

Failsafe events changed to errors so they are more obvious.
Errors recorded to dataflash for failure to init compass and optical flow sensor.
Errors recorded for pwm failure.
Resolved a compile error when dataflash logging is disabled.

Note: not yet enabled as part of any flight mode