Resolve "Add a Finite Volume local operator for the Richards equation"

What does this MR do?

Implements the finite volume scheme for Richards equation in a local operator, and adds it to the regular DORiE workflow by using the keyword richards.FEorder=0.

If the Richards FV solver is selected, the regular flux output is omitted. This is also mentioned in the docs.

Is there something that needs to be double checked?

• This MR cannot be accepted before !105 (merged). The reason is that gradients in FV are equal to zero, and hence, fluxes are meaningless here. !105 (merged) solves this reconstructing the fluxes of the FV scheme.
• I didn't implement source because is not currently utilized.

Can this MR be accepted?

• Implemented
• Documented (Doxygen)
• Documented (Sphinix)
• Added test
• Pipeline passing
• Added entry to CHANGELOG.md

Related issues

Closes #137 (closed)