Add PC-AiR

[Will-Tyler]

Description

In this pull request, I implement the PC-AiR method as described in this paper. This includes the PC-AiR algorithm for partitioning samples into an unrelated set and a related set. The method, pc_air returns a block matrix with the principle components as the columns of the block matrix.

Testing

I add unit tests.

I manually inspected the documentation to confirm that it matches my expectations.