src/Systems/ - support code for aircraft systems

This directory contains support code for major aircraft systems,
including the static, pitot, electrical, and vacuum systems.  The file
system_mgr.[ch]xx contains a subsystem group that holds all of the
individual systems.  Every system should extend FGSubsystem, and then
should be added to the group in the FGSystemMgr constructor.

Eventually, there will be an XML configuration file to select what
system modules should be available, so that different aircraft (i.e. a
twin plane with two vacuum systems) can have appropriate support.