Modifying pastis poisson

[gesinecauer]

Integrating the changes we'd discussed:

• Removing redundant MDS utils functions from utils_diploid.py
• Renaming "pastis diploid" to "pastis poisson" (and "utils_diploid to "utils_poisson", etc)
• Minor updates to installation instructions

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@c8ad557). Click here to learn what that means.
The diff coverage is 2.91%.

@@            Coverage Diff            @@
##             master      #65   +/-   ##
=========================================
  Coverage          ?   14.89%           
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  Files             ?       48           
  Lines             ?     3767           
  Branches          ?        0           
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  Hits              ?      561           
  Misses            ?     3206           
  Partials          ?        0

Impacted Files	Coverage Δ
pastis/optimization/piecewise_whole_genome.py	0% <0%> (ø)
pastis/externals/iced/setup.py	0% <0%> (ø)
pastis/optimization/utils_poisson.py	0% <0%> (ø)
pastis/optimization/estimate_alpha_beta.py	0% <0%> (ø)
pastis/setup.py	0% <0%> (ø)
pastis/optimization/initialization.py	0% <0%> (ø)
pastis/optimization/load_data.py	0% <0%> (ø)
pastis/optimization/callbacks.py	0% <0%> (ø)
pastis/optimization/pastis_algorithms.py	0% <0%> (ø)
pastis/optimization/counts.py	0% <0%> (ø)
... and 16 more

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[NelleV]

LGTM. Thanks!