ENH:Speedup of XRD caclulations

Open username-removed-457173 requested to merge Avakyan/ase:XrDebye into master

The performance of XRD calculations using XrDebye class improved (thanks vvpryadchenko@sfedu.ru). SAXS calculation mode added.

However, the signatures of constructor and get() method were affected, as I suppose, they become more reasonable. Also added methods for plotting as saving of calculated patterns.