Continuous Integration

This MR establishes the CI build process for Windows via cygwin.

Furthermore, metadata was added (codemeta.json) and the release process was extended so that tagged versions will be archived in the RADAR research data repository and get a DOI there. This requires a two-step release process triggered by generating a tag pre-X.Y (for example pre-4.0) rather than X.Y directly. (Release versions should only be tagged in the master branch)

I tested it as far as possible before merging into the master branch but I suppose something will go wrong when creating the next release. Just let me know, I can look into it.