add neutron stype in calculation

[dragonyanglong]

diffpy.pdfgui/src/diffpy/pdfgui/control/calculation.py

Line 179 in be24a9f

pc.qmax = self.qmax

from diffpy.srreal.scatteringfactortable import SFTNeutron                                          

    pc = PDFCalculator(**cfg)
    if stype == 'N':
    	ntb = SFTNeutron()    
    	pc.scatteringfactortable = ntb
    r, gcalc = pc(diffpy_structure)

see post at https://groups.google.com/forum/#!searchin/diffpy-users/neutron$20cmi|sort:date/diffpy-users/7nu23l7OY8A/bTDu_a0nCAAJ