Add support for the Conforming Crouzeix-Raviart Element for Stokes

The conforming Crouzeix-Raviart element P_2^+-P_1^\text{disc} is a stable mixed pairing for discretising the Stokes and associated problems, see e.g.

• M. Crouzeix and P.-A. Raviart, Conforming and nonconforming finite element methods for solving the stationary Stokes equations I, Revue française d'automatique, informatique, recherche opérationnelle. Analyse numérique, 1973, 7, 33-75
• Rolf Krahl and Eberhard Bänsch, Computational Comparison between the Taylor-Hood and the Conforming Crouzeix-Raviart element, Proceedings of ALGORITMY 2005, 2005, 369-379
• Cedric Thieulot and Wolfgang Bangerth, On the choice of finite element for applications in geodynamics. Part II: A comparison of simplex and hypercube elements, 2024, preprint

Our plan is to support generation of forms and operators for the P_2^+-P_1^\text{disc} pair. Initially we will only consider the case of triangles in 2D. Here the P_2^+ part for the velocity component employs continuous quadratic Lagrange elements extended by a cubic bubble function.

1. implement a P2PlusBubbleSpace in HOG to support P_2^+
2. add support for generating forms and operators using P_2^+ elements; especially diffusion operators
3. add support for DG1 elements to allow using P_1^\text{disc}
4. implement form generation for divergence and gradient parts using the P_2^+-P_1^\text{disc} pair
5. implement operator generation for divergence and gradient parts using the P_2^+-P_1^\text{disc} pair

Operator generation for the mixed pair will require more work as it necessitates extending the concept of an elementwise operator to DG, see #19. Note, however, that our case here is simpler, as we do only need volume integrals and do not have flux coupling or anything like that.