[ENH][WIP] Min/Max Spanning Trees functionality

[jGaboardi]

This PR adds minimum and maximum spanning tree extraction functionality.

See #449

[codecov]

Codecov Report

Merging #459 into master will decrease coverage by 5.45%.
The diff coverage is 98.43%.

@@            Coverage Diff             @@
##           master     #459      +/-   ##
==========================================
- Coverage   97.85%   92.40%   -5.46%     
==========================================
  Files           3        3              
  Lines        1120     1172      +52     
  Branches      269      284      +15     
==========================================
- Hits         1096     1083      -13     
- Misses          6       68      +62     
- Partials       18       21       +3     

Impacted Files	Coverage Δ
spaghetti/network.py	94.16% <96.66%> (-3.27%)	⬇️
spaghetti/util.py	82.37% <100.00%> (-16.10%)	⬇️

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