Add support for power transforms

[brandonwillard]

This PR adds support for general power transforms and replaces #184. It uses a positive support in cases of ambiguity.

In its current form, it appears to work as a replacement for at.reciprocal, but the $Z = X^2$, where $X \sim \operatorname{N}\left(0, 1\right)$, test example fails.

• [ ] Fix and add more tests