Add flow constraints

[bentaculum]

Adds two modular constraints necessary for a network flow formulation, useful for example for single particle tracking:

• Symmetry: # incoming edges = # outgoing edges
• Minimum track length: Each starting track needs to contain at least the given number of edges. Currently only implemented for length 1, i.e. each node with a set appear indicator needs to have at least one outgoing edge.

Each constraint includes a functional test.

[codecov-commenter]

Codecov Report

Merging #34 (53553f4) into main (dc797d4) will increase coverage by 0.32%.
The diff coverage is 97.82%.

@@            Coverage Diff             @@
##             main      #34      +/-   ##
==========================================
+ Coverage   92.47%   92.80%   +0.32%     
==========================================
  Files          30       32       +2     
  Lines         718      764      +46     
==========================================
+ Hits          664      709      +45     
- Misses         54       55       +1     

Files Changed	Coverage Δ
motile/constraints/min_track_length.py	95.23% <95.23%> (ø)
motile/constraints/__init__.py	100.00% <100.00%> (ø)
motile/constraints/in_out_symmetry.py	100.00% <100.00%> (ø)

[bentaculum]

@funkey I am wondering what would be a nice way to generalize the MinTrackLength constraint to arbitrary length (currently only implemented for min_length=1, which is trivial). Is there a way around creating indicator variables for all possible tracks of min_length and coupling them to e.g. the edges?

[funkey]

I think you could introduce an integer variable for each edge that counts the number of edges since appear (a constraint needs to ensure that each edge's length integer has to be one more than the incoming one). Then you can add a constraint that only allows for disappear events to happen if the integer is greater equal a threshold. That should do it.