Add DataCite compliant metada.xml. Adjust AUTHORS.md

This PR adds a DataCite compliant metadata.xml file that contains metadata about VirtualFluids. There are a few things that should be reviewed:

• I've set Manfred Krafczyk (and the IRMB) as the project's creator. He is not referenced in the AUTHORS.md file, though.
• Students who contributed code to VirtualFluids as part of their Bachelor's or Master's dissertations are now credited in the contributors sections. Their contributorTypes are set to Researcher. We might want to find a better type. Note that the possible contributor types are limited to the ones given in the DataCite schema.
• The project's creation date is currently set to the year 2000 as that was the year with earliest publication about VirtualFluids I could find.

Students who contributed code are also part of AUTHORS.md now. Added students are:

• Aileen Brendel
• Lennard Lux
• Sven Marcus
• Anna Wellmann