Fix the system of suggestions in validphys / config

[scarlehoff]

Due to the way the new commondata is implemented (grouped by paper), the suggestions (that looks at the entries in the folder) when a name is incorrect don't work anymore.

Fixing it should be a question of looking into config.py / loader.py and making sure a list of all datasets is available.

The only important thing to take into account is that it cannot be done at import time since listing all available datasets now require looping over metadata.yaml files, parsing them and constructing the dataset names.