Add typing and tests to the plot module

[tlambert03]

does what the title says

[codecov-commenter]

Codecov Report

Merging #32 (e9273ad) into main (b8929ae) will increase coverage by 15.50%.
The diff coverage is 94.73%.

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@@             Coverage Diff             @@
##             main      #32       +/-   ##
===========================================
+ Coverage   77.12%   92.63%   +15.50%     
===========================================
  Files          28       28               
  Lines         752      692       -60     
===========================================
+ Hits          580      641       +61     
+ Misses        172       51      -121     

Impacted Files	Coverage Δ
motile/plot.py	93.87% <94.44%> (+93.87%)	⬆️
motile/costs/features.py	81.81% <100.00%> (ø)
motile/solver.py	96.96% <100.00%> (+3.45%)	⬆️

... and 20 files with indirect coverage changes

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[tlambert03]

side note @funkey

I played with making is so that the plot is the default ipython repr of the track graph:

... but removed it assuming it might be too much to do for a typical graph. But, it might be nice to make a draw() method on the track graph? it could take an optional solver argument to draw solution if not None?

[tlambert03]

going to merge this one. @funkey I don't think you'll object, it doesn't change any functionality, just adds tests and typing