Add doc for johnson_alahi_li_2016

[pmeier]

@jbueltemeier Please rebase from the current master. In #122 and #123 I've added the framework to "see the doc strings in action".

[codecov[bot]]

Codecov Report

Merging #118 into master will increase coverage by 0.0%. The diff coverage is 100.0%.

@@          Coverage Diff           @@
##           master    #118   +/-   ##
======================================
  Coverage    98.3%   98.4%           
======================================
  Files          38      38           
  Lines        1365    1363    -2     
======================================
- Hits         1343    1342    -1     
+ Misses         22      21    -1     

Impacted Files	Coverage Δ
pystiche_papers/johnson_alahi_li_2016/_loss.py	100.0% <ø> (ø)
pystiche_papers/johnson_alahi_li_2016/_nst.py	100.0% <ø> (ø)
pystiche_papers/johnson_alahi_li_2016/_utils.py	100.0% <ø> (ø)
pystiche_papers/johnson_alahi_li_2016/_modules.py	95.8% <100.0%> (ø)
pystiche_papers/li_wand_2016/_loss.py	98.7% <0.0%> (ø)
pystiche_papers/li_wand_2016/_pyramid.py	100.0% <0.0%> (ø)
pystiche_papers/gatys_et_al_2017/_loss.py	100.0% <0.0%> (ø)
pystiche_papers/ulyanov_et_al_2016/_nst.py	95.6% <0.0%> (ø)
pystiche_papers/ulyanov_et_al_2016/_data.py	100.0% <0.0%> (ø)
pystiche_papers/ulyanov_et_al_2016/_utils.py	100.0% <0.0%> (ø)
... and 6 more

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[pmeier]

Test failure is unrelated. www.wassilykandinsky.net seems to be down.