CI: Replace minimal-ubuntu job with ubuntu

Closes pystencils#19

See merge request !22

- works on newer VS compiler locally
- comment this test out until CI system is properly updated

- test_serial_scenarios already tested if waLBerla is available by catching ImportErrors
- importorskip from pytest throws another exception, which is not part of the public API
-> catching all exceptions - which is not really elegant but works

- also for default MRT methods its better to use symbolic RR's instead of inserting numeric values for them
- but then the simplification strategy developed for SRT/TRT has to be disabled
- this commit introduces two new options:
     - simplification='auto' or a custom SimplificationStrategy
     - 'keep_rrs_symbolic'=True, by default now RRs are left symbolic

Execute LBM scenario with OpenCL

See merge request !13

Add custom timeloop_creation_function for LatticeBoltzmannStep

See merge request !20

Skip some tests if waLBerla is not installed

See merge request !18

update of the kernel equations for the phase field model

See merge request !19

minor documentation fix for Shan-Chen

See merge request !21

are much faster in certain setups

With this update all derivatives are derived automatically with the functionality of pystencils.
Furthermore more testcases are provided.

- more time steps to get to stable solution

Use Gram-Schmidt for create_mrt_orthogonal

Closes #5

See merge request !17

- "mrt" now takes relaxation rates [shear, bulk, 3rd order, 4th order..]
- deprecated "mrt3": use orthogonalized mrt now
- fixed wrong test

shear previously ignored the diagonal moments and bulk didn't exist

- orthogonalize automatically if no moments from literature are provided
- must specify whether weighted or unweighted orthogonality is desired