Add Sanakoyeu 2018 gan loops

[pmeier]

Blocked by #176 and #189. I'll review after these are merged.

[jbueltemeier]

I will add the tests later when the loops are ready.

[codecov[bot]]

Codecov Report

Merging #192 into sanakoyeu-2018 will decrease coverage by 2.5%. The diff coverage is 20.2%.

@@               Coverage Diff                @@
##           sanakoyeu-2018    #192     +/-   ##
================================================
- Coverage            97.9%   95.4%   -2.6%     
================================================
  Files                  44      45      +1     
  Lines                2046    2115     +69     
================================================
+ Hits                 2005    2019     +14     
- Misses                 41      96     +55     

Impacted Files	Coverage Δ
pystiche_papers/sanakoyeu_et_al_2018/_nst.py	19.1% <19.1%> (ø)
pystiche_papers/sanakoyeu_et_al_2018/__init__.py	100.0% <100.0%> (ø)

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[pmeier]

@jbueltemeier If you are happy, you can merge this. I'll add tests later myself.