dplyr::arrange makes things easier to read
Reorg'd unit tests.
Added some unit tests for HDRDA. #29

One test fails because I discovered some issues with
(lambda, gamma) = (0, 0).
Better handling of when HDRDA's tuning parameters are both 0.

Also, corrected calculation of W_k and Q_k in HDRDA classifier.
Added log-determinant helper function.
Tests for specific quantities within HDRDA classifier. #29