structureViewer for magnetic structure

[dragonyanglong]

@benfrandsen The default structureViewer to visualize the crystal structure is at this file: https://github.com/diffpy/diffpy.pdfgui/blob/master/src/diffpy/pdfgui/control/structureviewer.py

To set it up in the pdfgui UI, people can refer to this post: https://groups.google.com/g/diffpy-users/c/MWBU9kpUinA?pli=1 Such as VESTA etc. So I guess it is likely to work for .mcif file etc as well if call this API (the visualization button next to the gear) from PDFgui.