Add an example comparing mega- and meta-analysis

[tsalo]

Closes #59.

Changes proposed:

• Add an example comparing meta- and meta-analysis.
• Add statsmodels as a dependency for the documentation group.

To do:

• Plot study-wise and general effects and variances.
• Show how mega-analysis is superior to meta-analysis. The example dataset isn't great for that since the grouping of observations into "studies" is really arbitrary.

[codecov-commenter]

Codecov Report

Merging #74 (d2081c5) into master (9b8259b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #74   +/-   ##
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  Coverage   88.63%   88.63%           
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  Files          10       10           
  Lines         774      774           
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  Hits          686      686           
  Misses         88       88           

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[tsalo]

@angielaird have you come across any non-imaging datasets that could be useful for comparing mega- and meta-analyses?

[angielaird]

No, nothing comes to mind. Perhaps @nicholst can provide pointers?