WIP: Make DiscreteGlobalBasisFunction return values in world coordinates

Previously, DiscreteGlobalBasisFunction objects return function values as they are handed out by the LocalBasis objects. This is always the coordinate system of the reference element, which is not what is desired. This does not pose a problem for affine families of finite elements, because there the two coordinate systems coincide. However, for non-affine FE like Raviart-Thomas finite elements, an additional transformation is missing (the Piola transform in case of the RT-FE). This patch introduces a callback in the DiscreteGlobalBasisFunction for the correct transformation, and implements it for the RaviartThomasBasis implementation.