Extended section on dataset aggregation

[billschereriii]

Expanded the documentation section on DataSet aggregation and moved it to a new Advanced Topics page

[codecov]

Codecov Report

Merging #390 (da57605) into develop (ea93e2f) will increase coverage by 0.02%.
The diff coverage is n/a.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #390      +/-   ##
===========================================
+ Coverage    78.32%   78.35%   +0.02%     
===========================================
  Files           97       97              
  Lines         7050     7058       +8     
===========================================
+ Hits          5522     5530       +8     
  Misses        1528     1528              

see 8 files with indirect coverage changes

[ashao]

Some very minor comments, but otherwise thank you for expanding this section!

[mellis13]

Thanks for improving the docs! Just a couple of small questions/comments.

[ashao]

I agree with the changes you made in response to Matt's comments. Thanks again!