DataFlash: add EnableWrites method

This method allows temporarily suspending writes to the dataflash which
will be used in ArduCopter to ensure no messages are written while the
copter is disarmed
Copter: suspend logging while disarmed
Copter: remove armed check before logging
Merge remote-tracking branch 'upstream/master'
OptFlow: shrink lib by removing unused functions

Saves 46bytes of RAM and 1k of flash
OptFlow: update example sketch to work with shrunken library
Copter: integrate shrunken optflow
Plane: Fix parameter documentation

Fixes some problems with incorrect docs which would in turn generate
bad amp.pdef.xml files for ground stations.

merge with below
Merge remote-tracking branch 'upstream/master'
AP_Arming: param doc fixes

These were confusing the parser, or just incorrect
TradHeli: disable AutoTune for Tradheli compile

We need the flash space, and this probably won't work for tradheli anyway since tradheli needs most of the control from Feedforward rather than P term.  And D-term is very very bad.
TradHeli: enable Landing Collective when we are not moving

This prevents full negative pitch when touching down in Alt Hold or Auto-Landing.
TradHeli: rename Stab_Col parameters

With H_ prefix they appear with the rest of the tradheli specific params in the parameter list.
TradHeli: Minor change to two param defaults

Also one param name in the code clarified.
TradHeli: Drift mode to use heli manual throttle
Merge remote-tracking branch 'upstream/master'
AP_GPS: Improved accuracy of NMEA driver

The conversion of ret (32bit-integer) to float reduced accuracy to ~9cm or ~22cm. Now it's ~1cm.
Rover: try changing GPS setting to AIRBORNE_4G

this may reduce the GPS latency and improve chicane performance
HAL_PX4: fixed build for new PX4Firmware
Copter: disable OPTFLOW for PX4/Pixhawk