API Changes

Remove NMatrix#getrf: This one doesn't make sense, because it's not really analogous to #getrf!, which returns the pivot array, and replaces the original matrix with LU. #getrf just returns the LU part and throws away the pivot array.

Remove NMatrix#lu_decomposition!: This is an alias for #getrf!, but the name seems too similar to #factorize_lu which does a different thing.

Change #getrf!: The clapack_?getrf interface is strange and doesn't match the LAPACK spec. Make #getrf! match LAPACK rather than ATLAS CLAPACK. All the callers were confused about this anyway, so I think it's OK to change. This has the downside

Change #factorize_cholesky: This version assumes that the matrix is symmetric and so only has to call #potrf! once. The old version called #potrf! twice, and did not really make sense.