Fix types and linting; formalize typing of Node ids and Edge ids

[tlambert03]

This elaborates on the typing of Node and Edge ids, (and the keys that are valid when indexing into a variable).

Couple specific things to pay attention to:

1. the Variable class becomes class Variable(ABC, Mapping[_KT, int]): ... by implementing items and keys, etc... Variable was close to implementing the Mapping interface. This makes it official, and parametrizes the key type that a variable manages. All a subclass needs to do now is:

   class NodeSelected(Variable[NodeId]):
       @staticmethod
       def instantiate(solver: Solver) -> Collection[NodeId]: ...

   and then it will be clear why the index type needs to be when someone indexes later after using get_variables

2. This slightly relaxes the return type of instantiate_constraints from list to Iterable[ilpy.LinearConstraint]... Which means that instantiate_constraints() can now yield constraints (rather than having to build its own list which will then get iterated again in Solver._add_variables

[tlambert03]

ready for review @funkey

[codecov-commenter]

Codecov Report

Merging #28 (261df1c) into main (b61f7a1) will decrease coverage by 0.45%. The diff coverage is 75.43%.

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@@            Coverage Diff             @@
##             main      #28      +/-   ##
==========================================
- Coverage   77.92%   77.48%   -0.45%     
==========================================
  Files          28       28              
  Lines         761      755       -6     
==========================================
- Hits          593      585       -8     
- Misses        168      170       +2     

Impacted Files	Coverage Δ
motile/_types.py	0.00% <0.00%> (ø)
motile/costs/edge_selection.py	94.11% <ø> (ø)
motile/plot.py	0.00% <0.00%> (ø)
motile/variables/edge_selected.py	80.00% <80.00%> (-8.89%)	:arrow_down:
motile/variables/node_selected.py	80.00% <80.00%> (-8.89%)	:arrow_down:
motile/track_graph.py	86.20% <83.33%> (-1.44%)	:arrow_down:
motile/variables/node_split.py	93.54% <85.71%> (-3.33%)	:arrow_down:
motile/variables/node_appear.py	95.45% <88.88%> (-2.33%)	:arrow_down:
motile/variables/variable.py	65.71% <91.66%> (-0.96%)	:arrow_down:

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[funkey]

Thanks a lot! :)