Add planarity test and PMFG

[josephcbradley]

This is an implementation of a planarity test and an algorithm to construct a planar maximally filtered graph, as mentioned in #153.

It is functioning and there are some basic tests for the planarity algorithm, but there are quite a few todos:

• add tests for the PMFG algorithm
• Speed up the PMFG algorithm (currently reallocating lots of structures that don't need to be)
• implement traits for the PMFG algorithm (I could not get this to work for the life of me, would appreciate a pointer from someone more familiar with the syntax!)

etc. etc.

This is my first pull request so feedback appreciated!

[codecov]

Codecov Report

Merging #208 (e2b4862) into master (53cb581) will increase coverage by 0.21%.
The diff coverage is 98.19%.

❗ Current head e2b4862 differs from pull request most recent head 9ea7ec8. Consider uploading reports for the commit 9ea7ec8 to get more accurate results

@@            Coverage Diff             @@
##           master     #208      +/-   ##
==========================================
+ Coverage   97.23%   97.45%   +0.21%     
==========================================
  Files         114      115       +1     
  Lines        6585     6752     +167     
==========================================
+ Hits         6403     6580     +177     
+ Misses        182      172      -10     

[simonschoelly]

Hey, thanks for this contribution, I think planarity is indeed something we could use.

It might take a bit longer to completely review this PR, as these planarity algorithms are not known that much, so at least I need to get familiar with them again.

[josephcbradley]

@simonschoelly I think that's everything for now.

I've added the docs for planarity and for the PMFG to the spanning trees page, as that's where it felt most relevant.