Merge branch 'feature/better-manufactured-solution-testing' into 'master'

Full test suite passes.

This fixes #29 

To achieve this goal, I did:
* switch to l2error comparison for all manufactured solution examples
* implement mixed expressions to allow that for systems
* fix `stokes_stress_symdiff`

The following issues remain, but the tests are deactivated until an upstream problem is fixed:
* `heatquation{_dg}_explicit` fails, new issue is #30.
* `poisson_dg_matrix_free` needs a suiting solver, depends on #31 kind of.

I also improved error handling: No more exceptions, instead non-zero error code. That way you get vtk output for failing tests (the previous behaviour was potentially harmful, as you could look at old solutions in debugging!!!).

See merge request !23