Use clang-format for formatting code

[sleweke]

Decide on a code formatting and write a clang-format description. This establishes clear formatting guidelines that can be applied as pre-commit hook. Also saves a lot of noise in the code reviews (e.g., formatting comments).

[schmoelder]

@jayghoshter Can I assign you to do this together with @Immudzen!?

[jbreue16]

We have decided to postpone this until some legacy branches are merged.
For new branches/PRs we stick to our new dogma of smaller PR's and fast, functional merges which will make it easier to rebase those once we merge this PR.

Hopefully we can merge this for version 5.1 (crystallization)

These are:

• Crystallization reaction module #112
• Oscillation suppression for DG #131 ? maybe not this one since its discontinued for now
• Add a DG discretization for a 2D LRMP with spatially constant parameters #160
• Add an exchange module for the MCT #257