Copter: fix @Group for SingleCopter
Copter: rename some parameter Display Names

Also move some parameter descriptions to @User Advanced
Fence: parameter display name fix
INS: move Product ID to @User Advanced
RC_Channel: parameter @Units to pwm
Copter: parameter @Units to Percent*10
Compass: slightly relax HMC5843 calibration limits

Some users have reported Z axis values as high as 1.33.  Their compasses
seem otherwise health so extending the limits slightly.
Copter: RCIN dataflash logging
Allow the SITL scripts to run on platforms without gnome-terminal

Introduce a new variable $APMTERM that specfies the terminal window
to spawn.  Defaults to xterm as the most commonly available option.

This changes the look and feel of the non-interactive terminals only so
most people won't notice a difference, but of course old behaviour can be
restored with

APMTERM=gnome-terminal sim_ardu...

or a bashrc export.
Add barometer drift parameter to the SITL library
Wire in random noise and fixed drift for the SITL barometer
Add GPS mixing to the Z axis to the IntertialNav library
Fix set and scale of home altitude
Dont use the GPS to correct the accel or vel estimates, its too laggy and rubbish for that. Use for 3rd order term only
Turn the maximum time constant for GPS back down to 30 seconds, this is already vanishingly small compared to any sane barometer gain
Re-enable the GPS mixing in to the velocity estimates, but normalise the gains for the two sources to avoid double counting errors
Add Rangefinder messages to the standard set of those sent, alongside GPS messages in the position stream
Calculate the gain normalisation whether or not we have several devices connected in the z axis.  The maths is still correct and this is neater
Wire in basic support for SITL downward-looking sonar sensors
Add noise and glitching behaviour for the sonar sensor