Began immutable `DestroyCell`.
Merge branch 'ImmutableCells' of https://github.com/nskinkel/309Project into ImmutableCells
Made `NetInfoCell` immutable.

Also made one last change to `DestroyCell`.
Implemented immutable `RelayExtend2Cell`.
Merge branch 'ImmutableCells' of https://github.com/nskinkel/309Project into ImmutableCells

Conflicts:
	oppy/cell/util.py
Made some fixes to `RelayExtend2Cell`.