Magnets: changed implementation of edge coloring

[brad-lackey]

Changed how edge coloring is implemented:

• Old: self.part was a list of lists indicating the partitions of the edges.
• New: self.color is a dictionary keyed on the vertices of the edge with values indicating the "color" or which part the edge belongs.
• Property ncolors gives the number of parts (colors) in the partition

Convention: self loops are all called "color" -1. Remaining colors are 0, 1, ....

• Automatic indexing (creating lists of parts) then places the self loops at the end (index -1).

Updated all the examples of graphs to using this coloring implementation

• Chain1D, Ring1D, Patch2D, Torus2D, and CompleteBipartiteGraph have only minor changes.
• CompleteGraph has a new implementation of edge coloring.

All test files updated.