NasalSys.[ch]xx:

NasalSys.[ch]xx:
  implement FGNasalModelData class for execution of XML <load> and <unload>
  scripts. modelLoaded() is called by the model loader, and the destructor
  on branch removal.

modelmgr.cxx:
tilemgr.cxx:
tileentry.[ch]xx:
  make scenery and custom objects run their Nasal scripts on loading
  and unloading. Let OBJECT_STATIC object not be cached.