Resolve "use DGPk instead of OPB finite elements"

What does this MR do?

This MR wraps the usual \mathcal{P}_k elements of dune-localfunctions into \mathcal{P}_k^{dg} for discontinuous galerkin methods.

• The mass conservation test is currently testing conservation of mass for simplex. I checked visually that its solution is discontinuous and equivalent to what the OPB did before.

• Times does not seem to have decreased when comparing the mass conservation test. It might be because of the small size of the test and because the Richards equation most of the time is spent solving the linear system.

Discontinuities of the mass conservation test. (jumps were exagerated using a very low penalty term).

Can this MR be accepted?

• Implemented
• !105 (merged) accepted
• Documented
• Pipeline passing
• Added entry to CHANGELOG.md

Related issues

Closes #117 (closed)