Clean up the ReadMe file, and data storage for HPC2020

• rename README.md to README-old.md
• move sources/data-coupler/how_to_run_mn4.ipynb to easily readable /README.md , include important information from README-old.md, add information on hpc2020 and include instructions from the CONFLUENCE page on the data coupler.
• add a central folder to store the static data on hpc2020 - without any of the source code
• provide a data.md5 file in tests/data with the md5sum of all files in the file storage
• refactor the location of scripts, config files and data (see below)
• add some documentation on the data coupler and toy_model yaml file structure, with an example for each