Automatic code formatting using pre-commit and clang-format

[sleweke]

Enables support for pre-commit and clang-format. Code is automatically formatted before committing.

Options for clang-format are documented here. I used this online preview for various clang-format settings. I'm still not fully satisfied, but it's a start.

I'm not sure how this PR will impact other PRs and their "mergeability".

[sleweke-bayer]

This would fix #99.

[schmoelder]

I like this! :)

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[ronald-jaepel]

I've updated the PR to the current master, updated clang and re-formatted everything to confirm nothing breaks.

The ExternalFunctionTemplate.cpp files are whitespace sensitive, so they can't be reformatted. This can be acchieved by renaming them to .cpp.in template "input" files, that are then treated properly by clang.

[ronald-jaepel]

To set up on windows:

• activate proper python env (conda or venv)
• pip install pre-commit
• pre-commit.exe install in the source code folder

To process all files with the new pre-committ workflow:

• pre-commit run --all-files