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

Instead return a bool that influences the return code

Switch all poisson examples to comparing l2 error



See merge request !22

Always enable timings



See merge request !21

Add time measure capabilities

Fixes Issue #23.

See merge request !19

Feature/ini options

* Read formcompiler options from ini files
* Have a systemtest macros allowing variation in formcompiler arguments
* Rewrite the test suite using it

Exhibits some more problems in our test suite.

This fixes #1.

See merge request !20

[cleanup] Make function names more consistent

Small name changes. I succesfully ran all tests.

See merge request !18