Magnets: revisions to (hypergraph) coloring

[brad-lackey]

Issue: different terms in a Trotter expansion may need different coloring of the edges. So the coloring is not an intrinsic property of the graph. We remove (edge) coloring of a graph to be a seperate class, and shift all the coloring functionality to this class. Hence a single graph can now support multiple different colorings.

• Major revisions to the Hypergraph class and introduction of the HypergraphEdgeColoring class
• Introduction of the the edge_coloring() function that wraps greedy_edge_coloring() for the base class
• Revisions of each of the graph subclasses to remove coloring and create an overloaded version of edge_coloring()
• Revisions of the docstrings across all these classes
• Minor changes to some string representations
• Update to all the unit tests

[msoeken]

Code looks good to me, but some tests are not passing (see actions tab for details).

[brad-lackey]

Problem fixed (there was nondeterminism in one of the tests).