feat: add get_selected_subgraph

[tlambert03]

one option to close #66

@funkey, would be nice to get your thoughts on this API. There are many ways and other places where this could be done (such as a motile.Solution object that is able to get selected, etc...)

also, please weigh in on method name. Not sure get_selected_solution is the best. cc @cmalinmayor

[codecov]

Codecov Report

Attention: Patch coverage is 68.75000% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 91.61%. Comparing base (aa49bf6) to head (5613843).
Report is 4 commits behind head on main.

❗ Current head 5613843 differs from pull request most recent head 7ec7976. Consider uploading reports for the commit 7ec7976 to get more accurate results

Files	Patch %	Lines
motile/solver.py	68.75%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #82      +/-   ##
==========================================
- Coverage   93.29%   91.61%   -1.69%     
==========================================
  Files          32       32              
  Lines         776      799      +23     
==========================================
+ Hits          724      732       +8     
- Misses         52       67      +15     

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[funkey]

Wonderful, this is very useful, thanks a lot! I'd suggest calling the function get_selected_subgraph to be a tad more precise (the solution can also contain other variables associated with nodes and edges).

[tlambert03]

yes, I prefer get_selected_subgraph as well