Fix possible bug in potrf wrapping: return a triangular matrix

The cholesky decomposition is supposed to be upper triangular. The wrapper for
Lapack's `potrf`, instead, filled the resulting matrix to make it symmetric.I wonder 
if this was not a copy/paste overlook, since I do not see a reason to return a symmetric cholesky. 
Am I missing something?

This patch also adds a test that the result of cholesky decomposition actually factorizes a random
definite positive matrix.