Replacing list for chain definition with namedtuple

As mentioned in ATR-19605, a new way of defining the chain properties in the menus is being investigated via a namedtuple. Temporary check in place in DictFromChainName.py regarding the various possible ways to define the chain, this will eventually be removed once the development has been completely migrated to TMMT.

Tagging @stelzer as the format namedtuple was his idea (I think it works quite nicely). Also tagging @jmontejo and @aranzazu as menu coordinators, @astruebi as she's working on moving MET to TMMT. Also tagging @tbold to keep you informed.