parametrize BasisCreator with Grid instead of GridView
When using a BasisCreator, like LagrangeBasis or TaylorHoodBasis, it is required that the GridView is a LeafGridView, since this is passed to the create() method in createGlobalBasis() of ProblemStat. So, instead of giving the user the freedom to pass a different GridView than the leaf one, in the MR is Creator is parametrized with the Grid and the GridView is fixed to LeafGridView.