Adding single cell module

[gwaybio]

Implementing the strategy outlined in #100 (comment)

[codecov-io]

Codecov Report

Merging #111 (4cd0896) into master (ac3ef68) will decrease coverage by 0.11%.
The diff coverage is 98.11%.

@@            Coverage Diff             @@
##           master     #111      +/-   ##
==========================================
- Coverage   98.89%   98.77%   -0.12%     
==========================================
  Files          41       42       +1     
  Lines        1628     1636       +8     
==========================================
+ Hits         1610     1616       +6     
- Misses         18       20       +2     

Impacted Files	Coverage Δ
pycytominer/aggregate.py	88.46% <66.66%> (-9.71%)	⬇️
pycytominer/cyto_utils/cells.py	98.90% <98.90%> (ø)
pycytominer/tests/test_cyto_utils/test_cells.py	100.00% <100.00%> (ø)

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[gwaybio]

todo: add test to pycytominer.cyto_utils.cells.py in merge_single_cells method