Enable cell counting with more than 2 defined strata

[gwaybio]

Closes #75

[codecov-io]

Codecov Report

Merging #76 into master will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #76      +/-   ##
==========================================
+ Coverage   97.38%   97.52%   +0.14%     
==========================================
  Files          40       40              
  Lines        1795     1817      +22     
==========================================
+ Hits         1748     1772      +24     
+ Misses         47       45       -2     

Impacted Files	Coverage Δ
pycytominer/tests/test_aggregate.py	100.00% <ø> (ø)
pycytominer/aggregate.py	98.16% <100.00%> (+1.83%)	⬆️
pycytominer/tests/test_aggregate_profiles.py	99.19% <100.00%> (+0.17%)	⬆️

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