Added a class for GPU matrices

Added an SGMatrix-like class for GPU matrices. It supports conversion to and from SGMatrix and Eigen3 matrices, wrapping a matrix around pre-allocated memery, and serialization.

I've also wanted to add methods to SGMatrix that convert to and from Eigen3 matrices, however it fails to compile on travis for some reason.

@lisitsyn, @lambday, @karlnapf Please take a look.
@khalednasr I think we'd soon be able to have shogun::linalg::prod which does it for all :)