Add tools for symmetric bidigraphs

[stecrotti]

After all i think all the new stuff in #28 is not really necessary.

This PR is a thinner alternative. It implements:

• A function bidirected_with_mappings which takes an undirected graph and builds the corresponding bidirected graph. In addition it returns two mappings linking the indices of the edges of the two graphs. This turns out useful when you have some properties of the undirected edge (i,j) but you need it while looping on the directed edges.
• A method issymmetric(g::IndexedBiDiGraph) that checks whether the graph has the property that for each edge i=>j there always exists the parallel edge j=>i. There's no need for a new type.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.09%. Comparing base (fe7aaa7) to head (4d2968a).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #29      +/-   ##
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+ Coverage   83.08%   84.09%   +1.00%     
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  Files           8        8              
  Lines         331      352      +21     
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+ Hits          275      296      +21     
  Misses         56       56              

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