Solve submatrix system in the Schur complement
The non-diagonal elements in the sub-matrices in the Schur complement is
no longer ignored. Instead of assuming the matrix do be diagonal, and
compute the invert of the sub-matrix, small linear systems are solved
using superLU.

Tested on SPE3 and Norne. (With this fix a slightly modified norne runs
until 3292 days)
Change the maximum number of linear iteration
If we need more than 150 linear iterations, it's probably something
wrong, and we may be better of by restarting with smaller time-steps.
TODO: make it possible for the user to specify this number.
Adressing comments in the PR
1. The right hand side is solved only once
2. The solver is constructed directly with the matrix
3. const is added where it was missing
4. More commennts is added
5. Variable names are changed for clarification
Merge pull request #230 from totto82/fixSchur

Solve sub matrix systems in the Schur complement