for every edge, is it folded? or does it have
the potential to be folded? where "unassigned" is yes.
Description
For every edge, give us a boolean:
"true" if the edge is folded, valley or mountain, or unassigned.
"false" if the edge is not folded, any other assignment.
"unassigned" is considered folded so that an unsolved crease pattern
can be fed into here and we still compute the folded state.
a FOLD object