no `sratio`, `rcut`, 'stepcut` in CMI PDFCalculator

[dragonyanglong]

In pdfstructure.py,
getvar method can get the following values:
pscale, spdiameter, stepcut, delta1, delta2, sratio, rcut,
lat(n), where n = 1..6, x(i), y(i), z(i), occ(i), u11(i),
u22(i), u33(i), u12(i), u13(i), u23(i), where i=1..Natoms

sratio, rcut, 'stepcutare not available in PDFCalculator. Onlystepcutwhen useStepCutEnvelope`
http://www.diffpy.org/diffpy.srreal/api/diffpy.srreal.html#module-diffpy.srreal.pdfcalculator