SPARSE_SCHUR + CX_SPARSE = Faster

There was a bug in the trust region preprocessor where no fill
reducing ordering was computed for the case of SPARSE_SCHUR + CX_SPARSE
but this was not signaled to SchurComplementSolver, so it was using
a naive/natural ordering. To fix this two changes are made:

1. TrustRegionProcessor's logic for signaling the ordering to the
   linear solver has been re-worked. The surrounding code has also
   been re-organized for better readability.
2. In SchurComplementSolver::SolveReducedSystem the row and column
   block structure has been added to the CompressedRowSparseMatrix
   containing the Schur complement so that block AMD can be used.

As a result of these changes the linear solve time for
problem-744-543562-pre.txt has been brought down from 58 seconds to
35 seconds.

Change-Id: I4d82efce05175260f97b1f925f8a1b4a9d650cae