Add auto formatting and pre-commit

[jbreue16]

As per discussion in #402

Todo

• Fixes Use clang-format for formatting code #99
• Mention this in contributing.md, see https://github.com/cadet/CADET-Project-Management/issues/91

[schmoelder]

Note, for now the PR only includes a .pre-commit file. This only performs a very limited number of formatting steps (e.g. removing trailing whitespace). clang-format still would need to be configured (but could also be added to the pre-commit hook.

Note, we also need to add instructions on how to install pre-commit, e.g.:

sudo apt install pre-commit

and how to activate it:

pre-commit install

[schmoelder]

PR is probably also a duplicate of #148