Physical units in the GUI

[MBartkowiakSTFC]

By popular demand, MDANSE should have a way of selecting the units it will use for different physical quantities.

There is no need to change the way units are handled internally, but on the level of the interface we need to enable at least the units of energy to be chosen according to the preference of the user. Typically we should expect the neutron scatterers to use meV for energy, while conventional spectroscopy is likely to pick cm-1.

The preferred units should be used both for input and plotting.

[MBartkowiakSTFC]

This will be combined with issue #57 and solved by creating a general user settings object and a separate session manager.