FC.ParticleIsolation behaviour for CMULT=0

Hi all. Some thoughts on using the ParticleIsolation functorcollection. Probably relevant for @tfulghes

While trying to use collection, noticed that when CMULT=0 (i.e. when there are no particles in the cone), the sumcone momentum variables [CP, CPX, ..., CPT] become NaN. Feels more physically representative for this to be set to 0. Something like perhaps

    safe_sumcone = lambda func: F.VALUE_OR(0) @ F.SUMCONE(
        Functor=func, Relations=mu_isoAlg.OutputRelations
    )

Also, when using the ParticleIsolation variables, was given a new branch called indx, which as far as I'm aware isn't described anywhere. Thus I think if it has an intended use maybe add some information into the docstring for ParticleIsolation?