Add Jieping Ye's classifiers

[ramhiser]

At first, simply code up pure R versions. Later, it might be useful to use, say, an RcppArmadillo version for efficiency.

TODO: Make a list here of the various classifiers. For starters...

• Uncorrelated linear discriminant analysis (ULDA)
• Null-space linear discriminant analysis (NLDA)

[ramhiser]

Closing. Won't implement.