Moves regression-tests to tests/

- adds missing header to files
- unify header
- add SPDX (https://spdx.dev/) identifier to header (first step for getting REUSE https://reuse.software/ compliant)
- reworked doxgen

- uses awesome doxygen theme: https://jothepro.github.io/doxygen-awesome-css/

Adresses: #183

- adds wiki pages to doxygen page
- adds doxygen modules for production code and tests

Provide output folder as an artifact for the regression test to get access to the actual data set. Can be used for getting reference data as well.