property system: fix bug breaks inherited properties on splices
clean up the linear solver infrastructure

- Splices are now used more extensively
- The semi-implicit models now use the same linear solvers as the
  fully implicit ones. This has the pleasant side effects that the
  linear solvers are now properly separated from the models and that
  the semi-implicit models work in parallel. (if the grid provides
  either ghost cells or has an overlap of exactly one.)
- some renames for better consistency
add new reference solutions for the reservoir and the coupled tutorial tests

probably some more will be added tomorrow, when the testing cronjobs
are finished...