Finalized the _add_boundary method
- This now clones all 6 sides AND pushes the new pores to the periphery of the domain
- Had to make some changes to the clone pores method, it now DOES NOT clone labels.  It only applies the labels that are sent.
- Added a new private method to GenericNetwork called _reset_network which removes the adjacency and incidence matrices.  It is more efficient to remove them and rebuild once on demand, than to refresh them each time pores are cloned.