Add File Drawer Frequency algorithm to diagnostics module

[tsalo]

The File Drawer Frequency tool estimates the number of non-significant contrasts associated with a given large meta-analytic sample. We can implement this as a diagnostic that takes in a Dataset object and returns the predicted number of null findings that one would have included in the sample if not for publication bias. This may also be useful for generating and incorporating simulated null findings into meta-analyses to determine if meta-analytic results are robust to the file drawer problem. The latter application is mostly speculation, though.

[tsalo]

Related to #34.

[tsalo]

OSF Repo: https://osf.io/ayhfv/