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 (directly or via AbstractInstrument),
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.

Aircraft can select which instrumentation modules should be available
using an XML configuration file loaded as
<sim><instrumentation><path>.
An example of such a configuration file can be found in FGData at
Aircraft/Generic/generic-instrumentation.xml

