Solve sub matrix systems in the Schur complement

The non-diagonal part in the Schur complement computation is no longer neglected. 
Instead of computing the inverse explicitly, sub matrix systems are solved using superLU. 
Since this is only done for the well equations, the size of this sub matrices are small and the computational cost of solving the equations is small. 

Tested on SPE1, SPE3, SPE9 and Norne.  Do not change the results in the SPEs and and is necessary for running Norne. 
The name b used here is unfortunate, as in the comment above b is used (and is equation.value().matrix()).