Set upper Q limit of ~25?

[briantoby]

GSAS-II computations are inaccurate beyond sin(theta)/lambda of 2.0 (Q ~= 25 Å^-1) because the form factors are computed and fit only up to that threshold. Beyond there the form factors can be significantly wrong. [This might do bad things for x-ray PDFs as I(Q) is normalized with an averaged form factor].

To prevent GSAS-II from computing inaccurate intensities for ultra-high Q data or simulations, it might be good to prevent GSAS-II from computing x-ray patterns past Q of 25 Å^-1. To do this for powders, I can think of three code sections that will need to be touched:

1. When a powder pattern is read in, the 2theta max should not be allowed to exceed the Q cutoff
2. When limits are edited, the 2theta max should not be allowed to exceed the Q cutoff
3. Likewise pattern simulation should not exceed the Q cutoff.

X-ray single crystal reflection lists should also be trimmed of ultra-high-Q data (as if that will be collected)

TOF neutron data do not need this.