Acoustic phonon intensity - absolute unit validation for quartz

[rebeccafair]

As part of validation of the absolute intensity units in Euphonic, the intensities for Niobium were compared by hand to the acoustic phonon intensity in the long wavelength limit:

From equations 5.91 and 5.48 from Principles of Neutron Scattering from Condensed Matter, Andrew T. Boothroyd, Oxford University Press, 2020.

We should also do this for a multi-atom system e.g. quartz to ensure it is correct in these cases aswell.