Add edge_index_map() method to graph classes

[mtreinish]

This commit adds a new function edge_index_map() which will return a
mapping from edge indices to a tuple of the node indices and weight for
the edge.

TODO:

• Add tests
• Add release notes

[coveralls]

Pull Request Test Coverage Report for Build 883460719

• 195 of 205 (95.12%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.03%) to 95.508%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/iterators.rs	161	171	94.15%

Files with Coverage Reduction	New Missed Lines	%
src/isomorphism.rs	1	94.59%

Totals
Change from base Build 882184262:	-0.03%
Covered Lines:	5889
Relevant Lines:	6166

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💛 - Coveralls