Fix 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. Since this behaviour was introduced in commit 971973d0, which
dealt with C89 and added such lines for several wrappers, I wonder if
this was not a copy/paste overlook.

This patch also adds a test for the cholesky decomposition.
Test that Cholesky errs on rank-deficient input
Merge pull request #188 from jucor/TriangularCholesky

Fix possible bug in potrf wrapping: return a triangular matrix