MC25: Permute a sparse matrix to block triangular form

Permutes a sparse real or complex matrix to a block lower triangular form, i.e. given a sparse matrix \(\mathbf{A} = {\{a _{ij}\}} _{n \times n}\), it attempts to find permutation matrices \(\mathbf{P}\) and \(\mathbf{Q}\) such that \(\mathbf{\hat{A}} = \mathbf{PAQ}\) is block lower triangular and it returns \(\mathbf{\hat{A}}\) to the caller.