Implement RefinedLagrangeBasis in terms of the LeafPreBasisMapperMixin
Summary
As discussed in the original MR !305 (merged) it would be nice to simplify the index generation in the RefinedLagrange basis by using a mapper from dune-grid. Since the Mapper mixin class is provided in !426 (merged), this can now be done easily and is implemented in this MR. Note that for the refined P0 basis we assign multiple DOFs to the element.