fix how the RMS scattering angle is calculated

The RMS scattering angle calculation comes from Equation 33.15 in the PDG
article on the passage of particles through matter. It's not entirely obvious
if this equation is correct for a long track. It seems like it should be
integrated along the track to add up the contributions at different energies,
but it's not obvious how to do that with the log term.

In any case, the way I was previously calculating it (by using the momentum and
velocity at each point along the track) was definitely wrong.

I will try this out and perhaps try to integrate it later.

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