Remove symbol (didn't rebase the first time)

I didn't rebase the first time so I missed some new symbols comming from nonlinear jacobian apply.

See merge request !33

Remove symbol



See merge request !32

Cleanup/use using

Use using instead of typedef.

See merge request !30

[bugfix] Fix generation of the notebook script after dune-python refactoring

Requires latest dune-python.

See merge request !31

Requires latest dune-python.

Feature/nonlinear matrix free

Depends on PDELab merge request https://gitlab.dune-project.org/pdelab/dune-pdelab/merge_requests/183.

* [x]  stationary CG
* [x] stationary DG

Important notes:

- The nonlinear_matrix_free_dg_numdiff throws a time out. I already reduced the grid size to 4x4 and I don't want to go any smaler. Maybe we just delete this test? I think it's ok if we only test numdiff in CG since we want to use symdiff anyway. Opinions?
- Instationary matrix free needs some changes in PDELab. Should be postponed until it is supported there.


Closes #26 .

See merge request !29

Feature/stokes symmetrization

This fixes #10 

See merge request !25

Make dune-perftool python3-compatible



See merge request !28

and an example with stokes stress

Update to comply with refactoring of dune-python

Only merge once the upstream branch is merged.

See merge request !26

Feature/pandas integration

Add a script that allows to calculates performance regressions

See merge request !24

That allows us to load that file as a pandas frame.

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