Possibly erroneous fissile label for MG materials

[ChasingNeutrons]

It is possible that a non-fissile material may be labelled as fissile in MG. This can happen if fission XSs are provided but all equal to zero. It may be worth making the check for whether a material is fissile slightly more sophisticated to avoid possibly resulting divisions by zero.

[Mikolaj-A-Kowalski]

I think I am missing something, but in what context do we divide by the fission cross-section?

I just have a feeling that in the places where it is done we probably need to check that the cross-section is not zero explicitly before the division since a case where a value of fission cross-section is non zero for some energy ranges or groups and 0 elsewhere may be relatively common.

As for what to do in the case of all 0 fission cross-sections... I personally would be inclined to allow it but to emit a warning.