Block ordering for SPARSE_SCHUR + CX_SPARSE.

Uptil now only SuiteSparse when used with SPARSE_SCHUR would use
the block structure of the reduced camera matrix to find a fill-reducing
ordering.

This leads to substantial speedup for some bundle adjustment
problems.

Credit for this technique goes to the authors of g2o. I learned
about it from reading their source code.

Change-Id: I5403efefd4d9552c9c6fc6e02a65498bdf171584