Code format via clang-format

[rolandreichweinbmw]

Hi! When applying the included .clang-format to ETL's *.h files, considerable changes occur.

I.e. currently, the format is not consistently enforced.

I propose:

• (Optional) Agreeing on a new .clang-format to be applied generally
• Apply .clang-format to the whole codebase (ETL and tests)
• Check in CI gate via clang-format if changes from then on adhere to the style guide defined by .clang-format