`domain_average` should default to rotations/reflections in the spacegroup symmetries

[kbarros]

The current interface for domain averaging requires the user to pass a list of rotations. Given that the crystal must be supplied in any case, should the default behavior be to average over all orthogonal transformations of the spacegroup symmetries? In the common case, this would simplify the interface to:

domain_average(f, cryst, qpts) do q
    # Calculate intensities
end

To do: Understand if this is correct, especially with respect to "weights".