added clang-format structure

[JensWehner]

This is the start to #28

The next step would be to pick a format specification, you like. This format is specified in the .clang-format file.

This can help to determine what you like. https://zed0.co.uk/clang-format-configurator/

Afterwards the whole codebase can be reformatted using a simple make format.

In a last step the CI could check the formatting and reject PRs, which are not properly formatted.
@junghans can probably comment on that.

[codecov]

Codecov Report

Merging #30 (ee988e0) into master (e6aa13d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #30   +/-   ##
=======================================
  Coverage   89.71%   89.71%           
=======================================
  Files          19       19           
  Lines        1954     1954           
=======================================
  Hits         1753     1753           
  Misses        201      201           

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[robashaw]

I'm closing this PR because I've taken what you've done here and put it in the upcoming update, see PR #51
Thanks for introducing me to this!