Separate combinatorial state from force and lengths

[arpastrana]

Instead of this:

TrailEdge(u, v, signed_length, plane=None)
DeviationEdge(u, v, signed_force)

Do this:

TrailEdge(u, v, state, length, plane=None)
DeviationEdge(u, v, state, force)

This would require adding a new attribute to the edges dictionaries, namely {"state": -1} where possible states are only -1 if the edge is meant to be in compression and +1 if it should be in tension.
Adjustments in force.py and force_numpy.py should follow.