Allow source particles with energy below cutoff

[paulromano]

This PR allows a source energy distribution to include particles below the user-defined cutoff (default 1 keV for photons). Right now, if you have a discrete distribution and some of the points are below the cutoff, you will get an error about the source energies. However, a more desirable behavior would be to allow source particles of these energies and just have them immediately deposit their energy. We already have an early check to kill off photons below the cutoff before any collision processing:

openmc/src/physics.cpp

Lines 269 to 277 in d7fc8e5

	// Kill photon if below energy cutoff -- an extra check is made here because
	// photons with energy below the cutoff may have been produced by neutrons
	// reactions or atomic relaxation
	int photon = static_cast<int>(ParticleType::photon);
	if (p.E() < settings::energy_cutoff[photon]) {
	p.E() = 0.0;
	p.wgt() = 0.0;
	return;
	}

So with that, the fix is as simple as allowing the source particles to be born in the first place.

This has come up in the context of fusion dose rate calculations where a decay photon source is generated from an activated material. The new decay_photon_energy function/method often results in a distribution that includes low-energy photons that will trigger the aforementioned error.

[shimwell]

This looks great, certainly a nicer way to deal with the low energy photons from these decays sources. I'm looking forward to giving this a go tomorrow.

[shimwell]

Super stuff just what we needed for the R2S work Ethan and Patrick are working on. Many thanks