Support combined single and multiple Coulomb scattering

[amandalund]

This refactors and updates the single Coulomb scattering code in preparation for adding the Wentzel VI MSC model and combined single and multiple scattering. The main changes are:

• Refactor most of the Coulomb scattering model data into a separate Wentzel OK&VI data and params class. This data will also be used by the Wentzel VI MSC model, which samples single scattering for small step lengths and needs the parameters for the quantities calculated in the Wentzel helper class (cross sections, screening factor, etc.)
• Sample the scattering angle and calculate the Coulomb cross sections for arbitrary min/max scattering angles instead of assuming polar angle limits for single scattering. With combined SS and MSC, Wentzel VI will be used to sample scatterings with angles below a given polar angle limit, and single Coulomb scattering will be used above the limit.
• Calculate the electron and nuclear cross sections in addition to just the xs ratio (Wentzel VI will need the total xs)
• Import the MSC theta limit, angle limit factor, and nuclear form factor type from Geant4

[amandalund]

Thanks @whokion for the helpful feedback! I agree about adding more testing, and I also still need to set the correct lower energy limit for the Coulomb scattering model, which I'm planning do in a follow-up PR.

[sethrj]

Thanks @amandalund ! @hhollenb Do you want to take a look at this before I merge?

[amandalund]

Thanks @hhollenb, that's great!