inccooperate R6 documentation with roxygen

[GregorDeCillia]

The development of roxygen2 now supports R6 classes and makes it possible to use inline documentation for methods (See r-lib/roxygen2#922 for details). This should be used in the future to document our classes and methods. The advantage here is that the documentation will be more consistent (internally and with other R packages) and we will get automatic warnings if parts of the public API are undocumented. More precisely, all public fields and methods need to be documented. Furthermore, all arguments of public methods also need to be documented (similar to functions).

TODOs:

• Document persephone and all derived classes through this mechanism
  • Possibly add feature requests or PRs to roxygen2 in case we need some extras (R6 support is not released on CRAN yet)
• Put the requirements for the dev version of roxygen2 into the DESCRIPTION file or mention it in the README.
• Set up travis to utilize the dev version of roxygen2 (if we change the DESCRIPTION this should be not necessary) and rebuild the documentation on travis

[GregorDeCillia]

@merangelik I'll familiarize myself with those new roxygen2 features in the near future and maybe assign some classes to you once we have some reference examples.

[GregorDeCillia]

Updates

• roxygen2 released version 7.0.2 on CRAN in december. This version incoperates the new r6 features. Therefore, we might not have to explicitly require the dev version of r-lib/roxygen2 via DESCRIPTION or travis.yaml
• a first version of the documentation is available in the r6_docs branch