This is a clean up commit prior to the subsystem API standardisation to simplify
the diffs. It includes all SGSubsystem and SGSubsystemGroup derived classes.
The extractBits(), signExtend(), and writeBits() functions of the anonymous
namespace in Input/FGHIDEventInput.cxx have been shifted out of the namespace
and are now exposed via the header. This is needed as
<Input/FGHIDEventInput.cxx> cannot be imported within the CppUnit
infrastructure, as it is already built into the run_test_suite binary.
