Feature/use custom geometry transformation for blockstructured

This computes the determinant and jacobian inverse transposed directly within loopy and does not call the corresponding grid functions. Using some simple precomputations this is faster if number_of_blocks>=2.

This also allows straight forward vectorization for unstructured grids.

I don't know how the computation of the geometry transformation is done in the sumfactored case, but maybe there is some overlap, which could be reduced.