General-purpose graph rewriting

[goodmami]

A useful feature would be to modify a corpus of graphs according to some patterns. (see also #2)

• prune leaf node or edge (e.g., prune udef_q, prune :polarity)
• prune branch (if any reentrancies are self-contained)
• treeify (sever reentrancies on serialization)
• prune branch and sever reentrancies
• replace node type or relation (sub def_q existential_q, sub :-EQ :MOD-EQ)
• insert relation and node (insert (a / *_n_* !:RSTR-H-of *) (a :RSTR-H-of (b / udef_q)))

[goodmami]

Renaming this as some transformations are already implemented in the penman.transform module, but not to the extent that this issue describes.

[goodmami]

Closing this as I won't be working on it, and it may be best as a separate utility.