Add Block AMD ordering for SPARSE_SCHUR + EIGEN_SPARSE.

Ordering routines for the Schur complement when using EIGEN_SPARSE.
Also integration into SchurComplementSolver.

Part of this CL is also a refactoring of the block jacobian matrix
construction.

Change-Id: I11d665cc7d4867c64190e6fed1118f4d2e13d59b