Instruments: explore the possibility to use multiple settings at once

[MatthieuDartiailh]

Currently the user can select a single setting to use for an instrument at a time. However this means that completely orthogonal settings (pyvisa configuration and conversion ratio of a magnet cannot be used at the same time, or not without making one inherit from the other which would be a pain).
I think it may be interesting to allow to select multiple settings as long as they do not conflict (the dictionary in which they store their values do not use the same keys).