Fix image sources

[jbueltemeier]

In #283, a licence information for the images of gatys_et_al_2017 from the original repository has been introduced. This information is generally useful for replications in which the images from the original repository are used.

For this reason, this PR integrates this licence information for all current replications.

[codecov-commenter]

Codecov Report

Merging #285 (2c0703e) into main (08b562b) will increase coverage by 0.0%. The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #285   +/-   ##
=====================================
  Coverage   96.5%   96.5%           
=====================================
  Files         39      39           
  Lines       1655    1660    +5     
=====================================
+ Hits        1598    1603    +5     
  Misses        57      57           

Flag	Coverage Δ
unit	96.5% <100.0%> (+<0.1%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pystiche_papers/li_wand_2016/_data.py	82.0% <ø> (ø)
pystiche_papers/gatys_et_al_2017/_data.py	100.0% <100.0%> (ø)
pystiche_papers/johnson_alahi_li_2016/_data.py	97.0% <100.0%> (+<0.1%)	:arrow_up:
pystiche_papers/ulyanov_et_al_2016/_data.py	88.4% <100.0%> (+0.2%)	:arrow_up:
pystiche_papers/utils/misc.py	90.4% <0.0%> (+0.1%)	:arrow_up:

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[jbueltemeier]

My intention to create a separate function from this was that this standardised sentence could be used in many replications.

Currently, this is only needed in two implementations, but could be needed in more replications in the future. Or alternatively for new implementations where the authors have integrated the images (perhaps in edited form) in their repo and can specify the original url.

It is therefore only intended to avoid duplicate code and to make the specification more consistent.