WIP: Resolve "Add time step schemes to the richards simulation"

What does this MR do?

Add time step schemes to the richards simulation

Implementing this MR:

Using the ode configuration for sand with polynomial order 1:

• heun, and shu3 methods do not work even for very relaxed numerical settings.
• rk4 method works with very small timesteps, they are so small that simulations take too much time to compute until the end.
• When compared against the ode solution fractional method had a higher flux error by one order of magnitude.

Therefore, the system test done here only will test implicitEuler, alex2, fractional, and alex3 against the ode solution for sand with relaxed flux errors for the fractional method. And this MR will still include heun, shu3, and rk4 just in case someone else wants to experiment.

Is there something that needs to be double checked?

Not yet.

Can this MR be accepted?

• Implemented
• Added test
• Added test to the pipeline
• Pipeline passing
• Added entry to CHANGELOG.md

Related issues

Closes #109