feat: Adds subset_multi method

[andres-ixpantia]

• Closes Add a mathod for subsetting a graph over many nodes #23

[codecov]

Codecov Report

Attention: Patch coverage is 94.52055% with 8 lines in your changes missing coverage. Please review.

Project coverage is 88.39%. Comparing base (65e9cab) to head (766b57a).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/directed/mod.rs	94.28%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #24      +/-   ##
==========================================
+ Coverage   87.52%   88.39%   +0.86%     
==========================================
  Files          12       12              
  Lines        1411     1551     +140     
==========================================
+ Hits         1235     1371     +136     
- Misses        176      180       +4     

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