Migrate flow algorithms from GraphsFlows

[gdalle]

Take everything in GraphsFlows.jl and put it in Graphs.jl.

This is half of the solution to #108

• remove the min cost flow part, which relies on JuMP.jl (see GraphsOptim.jl)
• copy-paste the code to src/flows
• copy-paste the tests to test/flows
• copy-paste the docs and put it in docs/src/flows.md
• modify src/Graphs.jl and test/runtests.jl and docs/make.jl to include necessary files
• format everything
• modify docs to include private names in a separate section
• modify nearly all tests to use generic graphs

@matbesancon since it's your code do you have anything to say about this?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5878e7b) 97.30% compared to head (b158054) 97.41%.
Report is 7 commits behind head on master.

❗ Current head b158054 differs from pull request most recent head 3ff0588. Consider uploading reports for the commit 3ff0588 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #329      +/-   ##
==========================================
+ Coverage   97.30%   97.41%   +0.10%     
==========================================
  Files         115      124       +9     
  Lines        6796     7259     +463     
==========================================
+ Hits         6613     7071     +458     
- Misses        183      188       +5     

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[gdalle]

Tests are failing because type inference in 1.6 is less good than in 1.10. They didn't use to in the original repo, presumably because of generic graphs. @simonschoelly maybe you could help?

[matbesancon]

I'm good with it yup thanks!

[gdalle]

This is the part that is not well inferred for GenericGraph

@traitfn residual(flow_graph::::IsDirected) = Graphs.DiGraph(Graphs.Graph(flow_graph))

[simonschoelly]

Can I suggest that we move everything initially to the Experimental submodule. This way

• We won't have any name clashes as long as we don't export anything.
• We don't initially guarantee that the API for flows is stable so we still can't change things
• It is easier to track what we change from GraphFlows.jl when we do this in a separate PR instead of doing it in this one, where we don't have a history.

[gdalle]

That's a good idea but as for tracking changes, beyond renaming I only did the bare minimum so that the code could run. A lot of initial changes are gonna be necessary even in the Experimental submodule

[gdalle]

Also I'm a bit lazy at the prospect of redoing all of these necessary changes from scratch just so we can track the renaming, but I understand your point

[gdalle]

Closing this to reopen a cleaner PR separating the copypaste from the actual changes to GraphsFlows