add random batch sampler

[pmeier]

This adds the ability to draw a predefined number of batches randomly from the dataset.

[codecov[bot]]

Codecov Report

Merging #237 (9884f14) into master (9113b5a) will increase coverage by 0.1%. The diff coverage is 100.0%.

@@           Coverage Diff            @@
##           master    #237     +/-   ##
========================================
+ Coverage    98.8%   98.9%   +0.1%     
========================================
  Files          38      30      -8     
  Lines        1512    1503      -9     
========================================
- Hits         1494    1487      -7     
+ Misses         18      16      -2     

Impacted Files	Coverage Δ
pystiche_papers/data/utils.py	100.0% <100.0%> (ø)
pystiche_papers/data/__init__.py
pystiche_papers/li_wand_2016/__init__.py
pystiche_papers/gatys_et_al_2017/__init__.py
pystiche_papers/johnson_alahi_li_2016/__init__.py
...ystiche_papers/gatys_ecker_bethge_2016/__init__.py
pystiche_papers/ulyanov_et_al_2016/__init__.py
pystiche_papers/utils/__init__.py
pystiche_papers/__init__.py

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