[wip] Avoid reallocation of local matrix

Resize local matrix once to localView.maxSize(). Then one does not need to resize is on each element. In fact each call to Dune::Matrix::setSize() leads to a reallocation, even if the size does not change.

There's an issue with a potential corner case: If a local assembler assumes, that the local matrix has exactly the localView.size() this will now fail.