lapack/gonum, mat: add Dpotrs and use it in Cholesky

Created by: vladimir-ch

Not really a necessary addition but I felt like adding something new to our lapack implementation and this looked easy enough to do within my free time constrains. Apart from that, I do think it is good to move as much of the computational stuff to lapack as possible and let mat only do the matrix handling around it.

Please take a look.