Adding unit tests for diploid

[gesinecauer]

I'm definitely not done adding unit tests, but figured I'd start this pull request so I can see the Travis CI builds

[codecov]

Codecov Report

Merging #66 into master will increase coverage by 20.77%.
The diff coverage is 31.57%.

@@             Coverage Diff             @@
##           master      #66       +/-   ##
===========================================
+ Coverage   14.59%   35.36%   +20.77%     
===========================================
  Files          48       48               
  Lines        3845     3798       -47     
===========================================
+ Hits          561     1343      +782     
+ Misses       3284     2455      -829

Impacted Files	Coverage Δ
pastis/optimization/utils_poisson.py	33.33% <ø> (ø)
pastis/optimization/multiscale_optimization.py	9.74% <0%> (+9.74%)	⬆️
pastis/optimization/callbacks.py	67.2% <100%> (+67.2%)	⬆️
pastis/optimization/initialization.py	62.19% <100%> (+62.19%)	⬆️
pastis/optimization/pastis_algorithms.py	34.69% <19.29%> (+34.69%)	⬆️
pastis/optimization/poisson.py	70.04% <30.76%> (+70.04%)	⬆️
pastis/optimization/piecewise_whole_genome.py	10.43% <50%> (+10.43%)	⬆️
pastis/optimization/constraints.py	35.91% <50%> (+35.91%)	⬆️
pastis/optimization/counts.py	43.24% <50%> (+43.24%)	⬆️
pastis/optimization/load_data.py	53.84% <80%> (+53.84%)	⬆️
... and 13 more

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