refactor: keep ilpy usage inside of Solver.solve [demo]

[tlambert03]

Here's a demo of what I was proposing in #60,
where the only usage of ilpy is now in the Solver.solve method itself... limiting direct dependence on the business logic of ilpy (things like std::vector<Constraint>) while retaining the performance where it really matters (in the solving)

(most of the lines here are just due to the copying of ilpy.Expression into this library)

👀 - note there is also breaking change here, where motile.variables.Variable is renamed to motile.variables.Variables (note the plural). So as not to conflict with a single variable in an expression, and to better represent that it is in fact a dict of multiple Variable instsance

[codecov]

Codecov Report

Merging #61 (4a7a6f6) into main (37b6067) will increase coverage by 0.78%.
The diff coverage is 97.08%.

@@            Coverage Diff             @@
##             main      #61      +/-   ##
==========================================
+ Coverage   92.65%   93.43%   +0.78%     
==========================================
  Files          32       33       +1     
  Lines         762      975     +213     
==========================================
+ Hits          706      911     +205     
- Misses         56       64       +8     

Files	Coverage Δ
motile/constraints/constraint.py	100.00% <100.00%> (ø)
motile/constraints/expression.py	92.10% <100.00%> (-0.40%)	⬇️
motile/constraints/max_children.py	100.00% <100.00%> (ø)
motile/constraints/max_parents.py	100.00% <100.00%> (ø)
motile/costs/features.py	81.08% <100.00%> (ø)
motile/solver.py	97.16% <100.00%> (+0.20%)	⬆️
motile/ssvm.py	100.00% <ø> (ø)
motile/variables/__init__.py	100.00% <100.00%> (ø)
motile/variables/edge_selected.py	100.00% <100.00%> (ø)
motile/variables/node_appear.py	100.00% <100.00%> (ø)
... and 5 more