VTD fusions

[pjrule]

We'd like to be able to reproducibly generate and apply fusions—constraints that force VTDs to always be allocated together. There are two current cases where this is useful:

• The allocation of island VTDs, which otherwise would be inaccessible from the mainland and thus inaccessible in the VTD graph
• The allocation of VTDs that are within other VTDs, which seems to happen a lot in Wisconsin for some reason

Some fusion functions were previously implemented in elbridge.fusions. This PR adds elbridge.graph.FusionGraph, which generates a fusion graph from a list of callable fusions and provides a fuse() method to apply fusions to a list of VTDs.

[codecov-io]

Codecov Report

Merging #2 into ci-cd will increase coverage by 0.57%. The diff coverage is 100%.

@@            Coverage Diff             @@
##            ci-cd       #2      +/-   ##
==========================================
+ Coverage   93.09%   93.67%   +0.57%     
==========================================
  Files          10       10              
  Lines         739      790      +51     
==========================================
+ Hits          688      740      +52     
+ Misses         51       50       -1

Impacted Files	Coverage Δ
elbridge/fusions.py	100% <100%> (ø)	:arrow_up:
elbridge/__init__.py	100% <100%> (ø)	:arrow_up:
elbridge/graph.py	96.34% <100%> (+1.3%)	:arrow_up:
tests/test_graph.py	100% <100%> (ø)	:arrow_up:

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