Joined edges: new "J" edge assignment

[edemaine]

FOLD requires faces to be split into subfaces for a few reasons:

• To specify face ordering information (two coplanar nonconvex faces can be ordered one way in one part and the other way in another part)
• To represent faces with holes

We want to keep track of these subfaces (faces in the FOLD structure) being part of a single intended/represented face ("superface"). The proposed simple way is to have a new edge assignment type, "J", for joined edge. F is an unfolded crease, and is meaningful from an origami perspective, whereas J edges are just for representation purposes.

Then we'll want a function that constructs the boundaries of a superface, by DFSing over J edges. Here we can get multiple boundaries, allowing for superfaces with holes. If the superface happens to be planar, we can characterize one as exterior and the others as holes. This effectively solves #21 in a simple way, so will likely replace #21.

Incidentally, J would be the natural assignment for triangulation edges in Origami Simulator (both for inputting manual triangulation and outputting with triangulation) — the equivalent of yellow lines in its SVG input format.

(discussion with @estollnitz, @origamimagiro, @amandaghassaei)