[installexternal.py] Use dune release 2.8 per default

See merge request !2927

Feature/improve staggered gridgeometry

See merge request !2901

* write scvfs directly into vector using global index and fixed local indices
* add flag in scvf for processor boundary

[1d3d][avg] Skip point source if the circle stencil is empty (zero coupling term)

See merge request !2923

In this case the integration element is zero anyways and there is no coupling  but the pressure may
become NaN and makes the simulation crash due to NaN residuals.

[readme] Remove 2->3 transition, clarify header

See merge request !2920

Fix/epsilon 2d 2d intersections

See merge request !2918

Dennis Gläser authored Nov 02, 2021 and Timo Koch committed Nov 02, 2021

This test is based on a failing case that occured during a simulation
intersecting two 2D-grids in 3D space.

[readme] Add information on running individual tests

See merge request !2915

[cleanup] Remove code deprecated before release 3.4

Closes #1094

See merge request !2908

Cleanup/dimless nums

See merge request !2917

[readme] Order build badges by importance for users

See merge request !2916