Part of merging !2201 in smaller parts.
- add the mass model headers
- add the energy model headers
- add mass-momentum coupling manager
- add Donea tests with new staggered (replaces old test)
Why is energy needed? -> implements same style as for pm models where non-isothermal version is also included in base model version. I don't see harm in adding this here already....
But maybe we should also add a NI test then.
Print error to analytical solution in Donea