Output uses the standard mesh output.

Add the mesh on which the flux is calculated to
the output section. Furthermore, it is necessary
to change the prefix because multiple meshes are
written.

- test data folders,
- tests names, and
- process names in the project files.

Rename the process type from GROUNDWATER_FLOW to STEADY_STATE_DIFFUSION.

- Rename to better reflect the contained date.
- Expected solution update due to output of specific flux instead of flux.