Release 2.9.1

See merge request !230

Prepare release 2.9.0

See merge request !215

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

The code of that implementation has been removed years ago.

It is not used!

It is not used.

It is not used.

It is not used anymore.

Remove unused files scan.h and scan.cc

See merge request !205

Run codespell during ci

See merge request !204

Found this because codespell (rightfully) complained
about the spelling.

It is not spelled correctly, and it is not used, either.