[ENH] Add function for aggregating donors

[rmarkello]

Adds a function for aggregating across individual donor microarray expression dataframes to aid in post-processing.

[codecov-io]

Codecov Report

Merging #25 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
+ Coverage   97.82%   97.84%   +0.01%     
==========================================
  Files          14       14              
  Lines         598      602       +4     
==========================================
+ Hits          585      589       +4     
  Misses         13       13

Impacted Files	Coverage Δ
abagen/process.py	100% <100%> (ø)	⬆️
abagen/__init__.py	100% <100%> (ø)	⬆️
abagen/allen.py	99.06% <100%> (ø)	⬆️

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[coveralls]

Pull Request Test Coverage Report for Build 135

• 5 of 5 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 97.841%

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Totals
Change from base Build 133:	0.01%
Covered Lines:	589
Relevant Lines:	602

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💛 - Coveralls