Move statistic conversion functions from NiMARE to PyMARE

[tsalo]

If we want PyMARE to be flexible, then I think we need to account for different types of data. NiMARE already includes a bunch of functions for converting a lot of different values, so I think we should just migrate those functions over here and add some examples using them.

The new datasets module should ultimately be able to provide us with a range of meta-analytic datasets that have different types of data that may need to be converted before we can put them in Dataset objects.

[tsalo]

Of course, the effectsize module automatically does a lot of these transforms, but users may want to handle the conversions manually.