pastis to pdb script w/ test cases

[mozesjacobs]

@NelleV Hi Nelle. This is my script for converting pastis to pdb, and I have some test cases as well.

[codecov]

Codecov Report

Merging #88 (5a28f72) into master (92e4f40) will increase coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #88      +/-   ##
==========================================
+ Coverage   48.56%   48.71%   +0.15%     
==========================================
  Files          54       56       +2     
  Lines        4341     4489     +148     
==========================================
+ Hits         2108     2187      +79     
- Misses       2233     2302      +69     

Impacted Files	Coverage Δ
pastis/optimization/negative_binomial_structure.py	91.66% <0.00%> (-0.84%)	⬇️
pastis/optimization/utils.py	34.48% <0.00%> (-0.14%)	⬇️
pastis/setup.py	0.00% <0.00%> (ø)
pastis/_dispersion/utils.py
pastis/_dispersion/__init__.py
pastis/validation/_realignment.py	0.00% <0.00%> (ø)
pastis/dispersion/utils.py	93.33% <0.00%> (ø)
pastis/validation/__init__.py	0.00% <0.00%> (ø)
pastis/dispersion/__init__.py	97.05% <0.00%> (ø)
pastis/optimization/negative_binomial.py	94.05% <0.00%> (+7.69%)	⬆️

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[mozesjacobs]

@NelleV I believe everything on this is fixed, including flake8.