Added FGEnvironmentMgr to provide information on the environment in

Added FGEnvironmentMgr to provide information on the environment in
different locations, and hitched it into FGGlobals.  FGEnvironmentMgr
has taken over as the subsystem, while FGEnvironment is simple the
information that it returns.  I've removed current_environment
completely -- everything now uses properties or goes through
FGGlobals.  FGGlobals itself has a couple of useful methods:

  const FGEnvironment * get_environment ();
  const FGEnvironment * get_environment (double lat, double lon, double alt);

The first one returns the environment data for the plane's current
position, while the second returns the environment data for any
arbitrary location.  Currently, they both return the same information,
but that will change soon.