Add positron correction for SB sampling

[sethrj]

This refactors the SB energy distribution into two parts:

• a non-templated component that sets up the energy sampling function and calculates the bounding cross section; and
• a component templated on a "correction factor" that samples an energy, gets the cross section at that energy from the non-template component and multiplies by a scaling factor (1 for electrons, the result of SBPositronXsCorrector for positrons), and rejects based on a random number and the maximum (scaled) cross section.

I added tests that demonstrate:

• nothing about the electron sampling changes
• the sampling efficiency does not change for a constant non-unity value for the scaling factor
• the sampling efficiency decreases slightly for positrons (since the scaled cross section shape is more pronounced/less flat)

[amandalund]

Nice, @sethrj! Looks good to me once the build passes.

[vrpascuzzi]

Looks good! +1 to @amandalund's suggestion.