adding support for F(Q) in sasmodels

[butler]

I(Q) is proportional to F(Q)² Hence our models return that. However there are times when it may be preferable to have access to the F(Q). For example it is required for the beta approximation (increasing the range of applicability for using most current interaction potentials) or if one wanted to use these as a library for building P(Q) for more complex shapes.

Providing a method to access the underlying F(Q) thus would be very useful.