Update to 2.11

See merge request !54

Improve the module description in README.md

See merge request !53

In particular, we include a few details and the pictures
from the dune-grid-glue module page on www.dune-project.org.
That page will be made to point to README.md directly
in the near future.

Fixes due to deprecated/removed functions

See merge request !52

fix

fix

This makes it clearer what we want to do.

This is what we test in the CI system.

add machine-readable copyright and license information

See merge request !51

See https://reuse.software/ for a description.

Install ringcomm.hh.

See merge request !50

Use HAVE_DUNE_UGGRID instead of removed HAVE_UG

See merge request !49

GridGlueVtkWriter: use range-based for to interate over intersections

See merge request !47

This also gets rid of the last mention of "RemoteIntersection(Iterator)".

use `0` as tag in MPI to avoid too large value

Closes #19

See merge request !45

[cleanup] use default member initializer

See merge request !46

Some versions of OpenMPI have a lower limit than `12345678`; the MPI
standard only guarantees values up to at least `32768` are valid.

Closes: #19

use [[deprecated]] instead of DUNE_DEPRECATED_MSG

See merge request !44

Merge branch 'use-std-optional' into 'master'

See merge request [!43]

  [!43]: Noneextensions/dune-grid-glue/merge_requests/43

They were needed when dune-grid-glue only worked with the sequential
version of dune-uggrid.

Merge branch 'cleanup-5' into 'master'

See merge request [!42]

  [!42]: Noneextensions/dune-grid-glue/merge_requests/42