Add periodic mesh support for GMSH; format everything a bit

[davschneller]

• Periodic mesh support for GMSH, msh2 and msh4.
• Format everything; add a pre-commit config
• Shorten and add the license headers akin to SeisSol (with extra treatment for the third_party folder).

[Thomas-Ulrich]

1. node version

with kubuntu 24, I get:

[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
An unexpected error has occurred: CalledProcessError: command: ('/home/ulrich/.cache/pre-commit/repo7ogbkjjg/node_env-system/bin/node', '/usr/bin/npm', 'install', '--include=dev', '--include=prod', '--ignore-prepublish', '--no-progress', '--no-save')
return code: 1
stdout: (none)
stderr:
    npm ERR! code EBADENGINE
    npm ERR! engine Unsupported engine
    npm ERR! engine Not compatible with your version of node/npm: markdownlint-cli2@0.18.1
    npm ERR! notsup Not compatible with your version of node/npm: markdownlint-cli2@0.18.1
    npm ERR! notsup Required: {"node":">=20"}
    npm ERR! notsup Actual:   {"npm":"9.2.0"}
    
    npm ERR! A complete log of this run can be found in:
    npm ERR!     /home/ulrich/.npm/_logs/2025-10-15T12_06_49_001Z-debug-0.log
Check the log at /home/ulrich/.cache/pre-commit/pre-commit.log

maybe it could be worth using an older rev v0.13.0?
(given that ubuntu 24 is the LTS, not that old).

EDIT: could update nvm with but do we want to force developers to do this update for a linter?

# Install nvm if not installed
curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/v0.39.7/install.sh | bash
source ~/.bashrc

# Install Node 20
nvm install 20
nvm use 20

2. Python version

Is this language_version specifier really necessary?
language_version: python3.13
I mean, we do not always have such a recent Python installed.

[Thomas-Ulrich]

LGTM. See also the 2 comments about precommits.

[davschneller]

Thanks for the feedback!

Regarding the pre-commit comments:

• node version: I tried to reproduce it on an Ubuntu 24.04 without a present node install; it worked there. Not quite sure what to suggest here tbh, as Ubuntu 24.04 should be able to somehow manage the node install I think. Otherwise—yeah, we can downgrade.
• Python version: yes; I'll remove it. (the pre-commit hook doesn't take care of that by itself somehow)

(all of these changes to the pre-commit config will also need to be propagated to SeisSol and a PSpaMM PR probably)

[Thomas-Ulrich]

ok then forget about the node version. maybe it is because I did several dist-upgrade instead of a new installation.