Follow-up from "WIP: Resolve "Couple the TransportSimulation with RichardsSimulation""
The following discussion from !96 (merged) should be addressed:
Check global mass conservation.
Similar to the mass conservation test of the Richards solver, we can run the coupled solver in multiple homogeneous and heterogeneous test cases and evaluate the solute mass conservation. Requires a separate test executable or a general check similar to the one implemented for the flux reconstruction.