Fix for inf loop neutron bgr sim

In neutron background simulation very long time (nearly infinite) are taken by transport of very low energy gamma. To suppress these gamma (they mainly produced by muons and pions in annihilation at rest) an extra tracking cut is added. By default all gamma a with E<100 eV are killed, for neutron background study all gamma below 500 eV are killed.

For neutron background study all ions with energy below 1 keV are killed as well.

This should not affect results of mainstream simulation but should protect from very low-energy gamma slow transport. This PR also include PR