Add mnemonic machinery to take into account mnemonics versions

This MR is here to solve #21 (closed)

• The mnemonics is a dictionnary, with str as keys and tuple of dict as values (instead of dict).
• A function mnemonics_for_run checks the extra_data DataCollection sources and keys to find which version of the mnemonic is available. The tuple structure ensures the order in which the check is performed, so that the versions can be sorted from most recent to least recent.

An example notebook is attached

Mnemonics_machinery.ipynb

Close #21 (closed) #12 (closed)