Cosine and Mahalanobis distance

This PR implements Cosine and Mahalanobis distances.
If you are concerned about pulling code, that doesn't help (depends on which particularly part of pulling that you are concerned about though), since you depend on mat64 for the interface definition. By specifying a RowViewer by method set rather than name I was just avoiding the mat64 dependency. If that's not actually an issue, just keep mat64.Dense.