Resolve "CFL condition is based on water flux"

What does this MR do?

Compute the apparent solute velocity in cfl_condition(). This requires both the flux and the water content grid functions, as \mathbf{v} = \mathbf{j}_w / \theta_w.

Additionally:

• Pass the FEM order as quadrature order to cfl_condition().
• Use the minimum corner distance of the cell instead of the cell diameter in the CFL conditon. This makes the returned value more conservative and robust against non-square grid cells.
• Reduce the default courant number and update its description.
• Improve errors and warnings regarding courant number input.

Is there something that needs to be double checked?

No.

Can this MR be accepted?

• Implemented updates to CFL condition
  • Calculate velocity from flux and water content
  • Use minimum corner distance for CFL condition
  • Update courant number error handling
• [ ] Added/Updated tests
• Added/Updated documentation
• Pipeline passing: https://ts-gitlab.iup.uni-heidelberg.de/dorie/dorie/commit/a928975ab36d1b030a6a57124d6273502d90d67f/pipelines?ref=172-cfl-condition-is-based-on-water-flux
• Squash option set
• Delete branch option set
• Added entry to CHANGELOG.md

Assignee: If the Squash option is set, check/update the commit message right before merging!

Related issues

Closes #172 (closed)