Expose sub-entity-type to Python interface

[jorgensd]

basix.cell.sub_entity_type is only available in C++:
https://github.com/search?q=repo%3AFEniCS%2Fbasix%20sub_entity_type&type=code

basix/cpp/basix/cell.cpp

Lines 379 to 400 in 0315c73

	cell::type cell::sub_entity_type(cell::type celltype, int dim, int index)
	{
	const int tdim = cell::topological_dimension(celltype);
	assert(dim >= 0 and dim <= tdim);
	if (dim == 0)
	return cell::type::point;
	else if (dim == 1)
	return cell::type::interval;
	else if (dim == tdim)
	return celltype;
	const std::vector<std::vector<std::vector<int>>> t = cell::topology(celltype);
	switch (t[dim][index].size())
	{
	case 3:
	return cell::type::triangle;
	case 4:
	return cell::type::quadrilateral;
	default:
	throw std::runtime_error("Error in sub_entity_type");
	}