for a little while, since it uses different properties. There are
some improvements, especially with searching and range. It also has
its own serviceable and in-range properties, independent of any
coupled VOR.
a spinning gyro.
Changed FGInstrumentMgr to inherit from FGSubsystemGroup, greatly
simplifying the (already simple) class. I should probably rename this
to FGInstrumentGroup or something similar, but not today.
Added the gyroscopic turn indicator (part of the TC).