Port structural probing

[john-hewitt]

This codebase could be a nice successor to the structural probes codebase except, well, it doesn't actually implement the specific structural probe data processors, probes, and reporters.

For example, the TwoWordPSDProbe and OneWordPSDProbe (link) need to be ported, the Task code for computing tree distances from conll files, the reporters for making the distance images and computing UUAS...

Because the code structure of the two repositories is very similar, this shouldn't be too much work -- and then all the benefits (e.g., easily hot-swapping different transformers models) would come to structural probes!