Formatting, Re-enabling the CI, Licensing Info and one Bugfix

[davschneller]

A refactoring PR with one tiny bugfix.

• We make the clang formatting style conform to the one of SeisSol. The clang-tidy style is still different however—but can be adjusted when needed.
• The unit tests are re-enabled. Also the tests for clang-format and clang-tidy are included.
• We conform to the REUSE specification now—i.e. every file has licensing information.
• Fixed a bug, where floating point numbers without an explicit sign after the d were not converted to a YAML-compatible float. As a result, now, 0d0 is now parsed correctly to 0e0 which is understood by YAML. Previously, it would have worked only for e.g. 0d+0.
• Add newlines everywhere.

[davschneller]

@ravil-mobile : is it OK for you to be credited in the copyrights like it is right now? (since you had it under your own Github account until this year)

(edit: got the "OK" on slack)