Add universal methods

[mtreinish]

In the rust generated Python API we need to have fixed class inpurts to
satisfy the traits used by the pyo3 macro generated FFI functions. This
results in duplicate methods like digraph_dfs_edges and graph_dfs_edges
with the same implementation just differing input types. To simplify the
API for users this commit adds universal functions to the python side of
the retworkx package to take in any retworkx graph object and dispatch to
the proper function in the rust generated api that relies on strict
input types.

Fixes #215

• Add release notes

[coveralls]

Pull Request Test Coverage Report for Build 528817992

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 95.684%

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Totals
Change from base Build 528817235:	0.02%
Covered Lines:	4456
Relevant Lines:	4657

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💛 - Coveralls

[lcapelluto]

This is so awesome!! 🎉
The only "oof" I have is: if you have to fix a typo in a docstring, or if you want to change a default value, you have to change it now in three places 😬

[mtreinish]

This is so awesome!! tada
The only "oof" I have is: if you have to fix a typo in a docstring, or if you want to change a default value, you have to change it now in three places grimacing

Yeah I couldn't figure out a way around that. :( We still need to export the per type functions both so they're accessible for the dispatch functions and for backwards compat. If we're going to be exporting the functions we'll want them to be documented.