Add NIST x-ray form factors and attenuation tool

[lemmatum]

These two sites can be used to lookup form factors and attenuation for UV, x-rays, and gamma-rays.

https://www.nist.gov/pml/x-ray-form-factor-attenuation-and-scattering-tables

https://physics.nist.gov/PhysRefData/FFast/html/form.html