Incorrect calculation of material collision stopping powers

[paulromano]

When we calculate electron/positron collision stopping powers (used for bremsstrahlung) for a material, we rely on Bragg's additivity rule to handle multiple elements, which is an approximation: https://github.com/openmc-dev/openmc/blob/d12e390f1a365c8e5b92e586cb8dc7a4599e80d2/src/material.cpp#L507-L509

To be fully correct, the collision stopping power would have to account for molecular properties like the mean excitation energy and the density effect correction (see discussion in this technical report).