remove `scattering_object` layer

[sbillinge]

Problem

its too complicated to import diffraction objects.

from diffpy.utils.scattering_objects.diffraction_objects import DiffractionObject

Proposed solution

get rid of the scattering_objects layer so it becomes

from diffpy.utils.diffraction_objects import DiffractionObject, PdfObject

and so on. If in the future we support non-diffraction objects we can move it out of diffpy.....

[bobleesj]

@sbillinge My understanding is that from diffpy.utils.diffraction_objects import DiffractionObject, PdfObject would require importing PdfObject and DiffractionObject from a single file of diffraction_objects.py. If my understanding is correct, I am thinking it might be difficult to maintain a single file.

How about we set up a folder system like below?

where we import PdfObject and DiffractionObject, etc. using

from diffpy.utils.objects.diffraction import DiffractionObject
from diffpy.utils.objects.pdf import PdfObject