apply clang-format

[Thomas-Ulrich]

apply clang-format on src and app, and add a GitHub workflow to check if the modified code is clang formatted.

[hpc4geo]

@Thomas-Ulrich This PR contains some non-mergable content. It appears your ci/workflow changes were applied after git submodule update --init was invoked. Hence the clang-formatting rules have been applied to yateto source, and now the yateto code wooers as new additions to the tandem repo.

Can you please resolve this?

What would be good is you only include the change sets for

format.sh
clang-format.yml

in this PR.

Then afterwards we can go and clean up the source tree applying the formatting rules.

[Thomas-Ulrich]

Right, I did add the updated yateto by mistake in the PR.
I now have 2 commits:
the first one: add the worfklow files.
The second one applying clang-format with:

pip3 install clang-format==19.1.3
.ci/format.sh $(which clang-format) .
git add -u
git commit -m "apply clang-format"

If you prefer you can cherrypick the first commit and run the 2nd command.

(note that I update clang-format to 19 instead of 18 (because it was giving roughly the same number of modified lines ~3000, so let's use a more recent version I thought).