Unify code for Raviart-Thomas polynomial space

[kronbichler]

Currently, we have the basic Raviart-Thomas polynomial space in

dealii/include/deal.II/base/polynomials_raviart_thomas.h

Lines 34 to 49 in c59c2cd

	/**
	* This class implements the <i>H<sup>div</sup></i>-conforming, vector-valued
	* Raviart-Thomas polynomials as described in the book by Brezzi and Fortin.
	*
	* The Raviart-Thomas polynomials are constructed such that the divergence is
	* in the tensor product polynomial space <i>Q<sub>k</sub></i>. Therefore, the
	* polynomial order of each component must be one order higher in the
	* corresponding direction, yielding the polynomial spaces
	* <i>(Q<sub>k+1,k</sub>, Q<sub>k,k+1</sub>)</i> and <i>(Q<sub>k+1,k,k</sub>,
	* Q<sub>k,k+1,k</sub>, Q<sub>k,k,k+1</sub>)</i> in 2D and 3D, resp.
	*
	* @ingroup Polynomials
	*/
	template <int dim>
	class PolynomialsRaviartThomas : public TensorPolynomialsBase<dim>
	{

and the nodal variant

dealii/source/fe/fe_raviart_thomas_nodal.cc

Lines 37 to 45 in c59c2cd

	// ---------------- polynomial class for FE_RaviartThomasNodal ---------------
	namespace
	{
	template <int dim>
	class PolynomialsRaviartThomasNodal : public TensorPolynomialsBase<dim>
	{
	public:
	PolynomialsRaviartThomasNodal(const unsigned int degree);

introduced recently for #12454. The latter is the more appropriate space for higher degrees because it uses non-equidistant points for the Lagrange polynomials, and it enables a renumbering. I think we should be able to move the code of the latter to the actual polynomial class, base/polynomials_raviart_thomas.h, and create an entry point based on the right numbers for all places the basic polynomial is used. This would reduce code redundancy. The only catch is that higher degrees with the previous PolynomialsRaviartThomas would change the form (into the better), so we might need to look into some tests.

[kronbichler]

Found while looking at #12726.