feat: allow indexing solution with Variable instance

[tlambert03]

Since Variable objects generally represent the index of some variable within a solver (and have an index attribute that returns the exact index), it makes sense to allow them to be used to directly index into a Solution instance. (i.e. to essentially return that .index attribute whenever the Variable is being used as an argument to __getitem__)

    solver = ilpy.Solver(5, ilpy.VariableType.Continuous)
    solution = solver.solve()
    x = ilpy.Variable("x", index=0)
    assert solution[x] == 0

[codecov-commenter]

Codecov Report

Merging #35 (abe4485) into main (818915a) will increase coverage by 0.03%. The diff coverage is 72.72%.

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@@            Coverage Diff             @@
##             main      #35      +/-   ##
==========================================
+ Coverage   87.43%   87.46%   +0.03%     
==========================================
  Files           4        4              
  Lines         366      375       +9     
==========================================
+ Hits          320      328       +8     
- Misses         46       47       +1     

Impacted Files	Coverage Δ
ilpy/expressions.py	90.54% <71.42%> (-0.63%)	:arrow_down:
ilpy/wrapper.pyx	82.72% <75.00%> (+1.24%)	:arrow_up: