Extended persistence

[sauln]

This PR has an implementation of the extended persistence.

It still needs some massaging, but the general process works on the few examples I've cooked up (see test_extended.py.

I would love it if anyone gave some feedback or did a code review.

TODO still:

• [x] Documentation:
  • [ ] more elaborate docstrings, and
  • [x] incorporate into sphinx docs.
• [x] Add an adapter for Kepler mapper. Should the adapter be housed here or in km?
• [x] Add an adapter for networkx.
• [x] Reorg api. The OOP interface that made sense for Alpha/Cech/Rips doesn't make quite as much sense here. Should we diverge, or come up with a unified interface?

[codecov[bot]]

Codecov Report

Merging #3 into master will increase coverage by 12.12%. The diff coverage is 88.49%.

@@             Coverage Diff             @@
##           master       #3       +/-   ##
===========================================
+ Coverage   79.61%   91.73%   +12.12%     
===========================================
  Files          10       12        +2     
  Lines         309      375       +66     
===========================================
+ Hits          246      344       +98     
+ Misses         63       31       -32

Impacted Files	Coverage Δ
cechmate/filtrations/custom.py	0% <0%> (ø)
cechmate/__init__.py	100% <100%> (ø)	:arrow_up:
cechmate/solver.py	97.36% <100%> (-0.04%)	:arrow_down:
cechmate/utils.py	100% <100%> (ø)
cechmate/filtrations/__init__.py	100% <100%> (ø)	:arrow_up:
cechmate/filtrations/extended.py	95.69% <95.69%> (ø)

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