Investigate convergence properties of sampling at spherical surfaces

[ajjackson]

Depends on #85

Consider strategies for identifying and accelerating convergence of S(q, ω) with respect to the number of q bins and the number of samples at each surface.

• Should/can we use adaptive sampling to focus on the more difficult areas of the phonon band structure?
• Can we develop an automatic convergence procedure to improve the user experience?
• How accurate/reliable are the results from "typical" settings compared to a fully-converged limit?

Significant developments in this area could merit an academic publication in their own right, so it would be worth doing a decent literature review first.

[ajjackson]

This Issue has effectively become duplicated/replaced by #116 . That issue number is referred to in more places, so closing this one.