Improve performance of CPR preconditioner.

This restores the performance to approximately the level it had before
the change to support non-diagonal well jacobians, for SPE9. All changes
are to the eliminateVariable() method.
 - Explicitly compute and apply the inverse.
 - Change loop ordering to apply inverse only num_eq - 1 times, instead
   of (numeq - 1)^2.
 - Use UmfPackLU instead of SparseLU.
Merge pull request #233 from atgeirr/improve-cpr-performance

Improve performance of CPR preconditioner.