[fix] Correct application of Piola transform to derivatives

The Piola transform applies the transposed of the Jacobian (among other operations). In the previous version, the plain Jacobian has been applied for sets of derivatives. This has been fixed.

FYI: With the current fix the example poisson-mfem.cc also runs for simplicial grids (not implemented in this merge request). If wished, I can enhance poisson-mfem.cc with the possibilty of using a simplicial grid.