- Ground network XML parsing code reads the new attributes "holdPointType"

- Ground network XML parsing code reads the new attributes "holdPointType"
  and "isOnRunway".
- Added initial support for AI controlled pushback operations, making use of the
  current editing capabilities of TaxiDraw CVS / New_GUI_CODE. The current
  implementation is slightly more computationally intensive than strictly
  required, due to the currently inability of taxidraw to link one specific
  pushBack point to to a particular startup location. FlightGear now determines
  this dynamically, and once we have that functionality in TaxiDraw, the
  initialization part of createPushBack() can be further simplified.
- Smoother transition from pushback to taxi. No more skipping of waypoints, and
  aircraft wait for two minutes at pushback point.
- The classes FGTaxiNode, FGTaxiSegment, and FGParking, now have copy
  constructors, and assignment operators.
- Removed declaration of undefined constructor FGTaxiNode(double, double, int)
- Array boundry checks and cleanup.
- Modified Dijkstra path search algoritm to solve partial problems. Currently
  limited to include pushback points and routes only, but can probably be
  extended to a more general approach.
- Added initial support for giving certain routes in the network a penalty, in
  order to discourage the use of certain routes over others.