Ensure secondary particles below energy cutoff are not created

[paulromano]

A user observed that in a photon transport problem, photons were being created (and contributing to flux) below the specified energy cutoff. The fix for this is simple -- I've added a check in Particle::create_secondary that prevents the secondary particle from being created if it's below the energy cutoff. Along with this, I've added a unit test that confirms the correct behavior, namely that no flux appears below the energy cutoff but that the total heating is not affected (the test uses an infinite medium problem, so the entirely of the source energy should be deposited regardless of the presence of an energy cutoff).

@amandalund since you're the expert with particle cutoffs/ secondary creation, would you mind reviewing?

[amandalund]

Looks good to me! Thanks for the fix and test @paulromano.

There are a couple places where we also have the check on cutoff energy in the physics model (bremsstrahlung, where it's needed to accumulate the energy carried away by secondaries; Compton scattering, where it's no longer required but prevents us from unnecessarily calculating the secondary direction). There are also places where we might consider adding the check in the physics (e.g. to avoid sampling the direction of a photoelectron that's below cutoff), but it probably won't make much difference performance-wise.