Two minor fixes

See merge request !3407

Newer compilers don't include cstdint implicit, we have to
do it explicit.

Feature/helmholtz operator

See merge request !3401

[preconditioners] Add three preconditioners

See merge request !3397

* Thread-parallel Jacobi smoother/preconditioner
* Thread-parallel SOR/SSOR with coloring
* They can be selected via the factory
* They can be used with AMG as smoother (but not unfortunately not via the factory yet)
The reason being that Dune currently hard-coded the selection of available smoothers.
We would have to write our own AMGCreator to include them. Probably better to find an
upstream solution.

Cleanup/partitioningyaspgrid

Closes #1224

See merge request !3394

[ci] Update the ci to use dune 2.9

Closes #1226

See merge request !3399

[solver] Use thread parallel linear operator

See merge request !3393

Provides some clearly measurable speedup for the modified test.
E.g. 1p_richards1p_1d3d, the solver time halves with four proceses.
Not a good as scalability of assembly but this is expected.

[disc] Add simple L2 projection implementation and test

See merge request !3390

New istl linear solvers

See merge request !3386

Triggered an error in UG

[parallel] Make parallel istl helper an implementation detail and specialize for grids with communication

[test][2p2c] Fix return type, const has no effect here

See merge request !3387

[assembler][pdesolver] Distinguish Resiudal and Solution types and set residual to dune native type

See merge request !3385