support sratio and rcut in cmi

[dragonyanglong]

In diffpy-cmi, there is no sratio and rcut functions available.

Sigma_ij = sqrt( sigma_isigma_j + f(delta1) + f(delta2) )
Sigma_ij is the width of the peak jointing atom i and atom j. Sigma_i is the adp of atom i.
If r_ij < rcut:
sigma_ij -> sigma_ijsratio
Ex: if rcut = 4.0, peak 3.95 A then we expand it via sratio so it gets to 4.1A.