Develop an improved PANTHER model

[ajjackson]

Looking at the ILL PANTHER website there are now some nice resolution function plots which I don't remember seeing before (and don't appear in the QENS/WINS paper): https://www.ill.eu/for-all-users/instruments/instruments-list/panther/characteristics

e.g.

If we get in touch with the instrument scientists it shouldn't be too difficult to turn this data into an improved model; I'd rather not keep unofficially digitising them from plots.