Investigate powder sub-shell sampling

[rebeccafair]

Currently, the sampling functions (and therefore sample_sphere_structure_factor and sample_sphere_dos) will only sample on the surface of a sphere at a specific |Q|, whereas it would be useful to be able to sample within a shell between |Q|_max and |Q|_min (i.e. at different radii). There are a number of approaches for this that need to be investigated.