Implemented new getCurrentMapNodesSpanned() code in AtlantisEntity.

Once the MotherShip sub class was added, the old routine turned out to
be inadequate, leaving gaps in the list of nodes spanned by the entity.
Minor changes to AtlantisEntity.

Commented out unused private method related to map nodes spanned
calculation. getCurrentMapNodesSpanned() now returns unmodifiable set.
Merge branch 'issue43' into develop