Add a Finite Volume local operator for the Richards equation
Description
As it was discussed in the work meeting, the Finite Volume (FV) method is interesting to have in DORiE for fast computations.
Proposal
Add a new local operator, pretty much as I have been doing with the transport solver: Swtich with template metaprogramming the grid function space and the local operators so that the same code for FV and DG is used.
How to test the implementation?
maybe testing against the ode solvers (?)
Related issues
See #...
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