Diagrams of C-Sets without repeated storage

[jpfairbanks]

Work with @kris-brown on this since he is currently refactoring a lot of Catlab to make features like this easier to implement.

[samuelsonric]

Here is an idea for graph decompositions.

(GE, GV, gsrc, gtgt) is the diagram shape, (SE, SV, ssrc, stgt) are the adhesions ("separators"), and (BE, BV, bsrc, btgt) are the bags.