Remove unused code from the `domain` directory

See merge request !217

I don't know what it used to be good for.  But nowadays it is not
used anywhere, and the state of all patches is always FIXED.

Hence let's just remove that code altogether.  It saves us one
integer per coarse grid boundary segment.

That method has been removed a while ago.  Apparently I forgot to
remove the declaration.

Adjust changelog to 2.9 release

See merge request !216

Reduce number of unused variable warnings

See merge request !214

Remove unused variabes.
Declare loop variable within for statement.

Update dune.module files after 2.9 release

See merge request !213

.gitlab-ci.yml: run "reuse" job in ".pre" stage

See merge request !212

add SPDX information

See merge request !211

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

DDD: add `DDD_InfoProcListRange` as a replacement for `DDD_InfoProcList`

See merge request !209

The `DDD_InfoProcList` function writes to a global per-grid buffer
causing issues in multi-threaded applications.

The replacement `DDD_InfoProcListRange` provides a range that only
looks up the requested data on demand without use of a buffer.

Reference: core/dune-grid!597 (comment 117843)

.gitlab-ci.yml: add "duneci" tag to spellcheck

See merge request !210

The shared runner might not pick up the job otherwise.

Feature/recude unused variables

See merge request !208

Miscellaneous cleanup patches

See merge request !207

... and purge a lot of obsolete stuff.

This makes the information easier to find.  It also allows
to avoid a number of redundancies.

It is not used.

[dddif] Initialize SonEdge before usage

See merge request !206