Try to coerce string input for subsample_n

[gwaybio]

closes #29

[codecov-io]

Codecov Report

Merging #34 into master will increase coverage by 0.03%.
The diff coverage is 76.47%.

@@            Coverage Diff            @@
##           master     #34      +/-   ##
=========================================
+ Coverage   95.87%   95.9%   +0.03%     
=========================================
  Files          25      25              
  Lines         751     757       +6     
=========================================
+ Hits          720     726       +6     
  Misses         31      31

Impacted Files	Coverage Δ
pycytominer/aggregate.py	88.18% <76.47%> (+0.68%)	⬆️

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