OPEN-PROM paper and online manual

The documentation required by DIAMOND has been published [online](https://e3modelling.github.io/OPEN-PROM/), see Issue https://github.com/e3modelling/OPEN-PROM/issues/646. The next steps are:

- Converted the DIAMOND deliverable into a a journal publication. 
- The website will be replaced by a "manual", generated from the inline comments and code, automatically updated. 