Further performance improvement by avoiding zeros in matrix-matrix product.

This PR is based on top of #248 and simply avoids zero enries in the matrix-matrix multiplication. 
However, the matrix structure is modified and this might not be wanted. On the other hand the speed up is significant (approx 40% for NORNE compared to master). The downside is a slightly higher number of iterations. 
The results for SPE9 are the same. The max_iter has to be raised though.