Simplify the code a bit by computing the dim-dimensional prolongation matrix by the product of the 1d matrix, rather than using the nodal points of the polynomials which only work for FE_Q and FE_DGQ.
This needs to be merged after #3770.
Allow for FE_DGQLegendre in MGTransferMatrixFree
OK once tester is happy.
The tester only complains about the missing `FE_DGQLegendre class which has been merged after the tester started running, so I'm merging now.