Conegation rewriting

[kris-brown]

Using the logic of biheyting algebras (described here) it should be straightforward to implement conegation rewriting (described here).

This semantics would be an intermediate between DPO (don't apply rule if it produces a dangling connection) and SPO (cascade delete until there are no dangling connections). It would still apply the rule, adding/deleting/merging things, except it will skip deleting anything in particular that would produce a dangling connection.

The corresponding PR could also implement the pushout complement code in terms of biheyting algebras (although the check if the dangling/identification conditions hold will remain unchanged).