src/Instrumentation/ - gauge and avionics support code

This directory contains code to support gauges, avionics, and other
instruments in FlightGear.  The file instrument_mgr.[ch]xx contains a
subsystem group that holds all of the individual instruments.  Every
instrument should extend FGSubsystem, and then should be added to the
group in the FGInstrumentMgr constructor.

Code is gradually moving into here from other areas, especially the
src/Cockpit/ directory.  Eventually, there will be an XML
configuration file to select what instrumentation modules should be
available, so that different aircraft can have appropriate support.