Ability to select energy range in cosmic generator

[lobis]

<generator type="cosmic">
    <source use="Cosmics" direction="(0,-1,0)" filename="../cosmics.root"
            particles="neutron" energy="(1,10)MeV"/>
</generator>

Works to limit the range of the generator, the equivalent simulation time is adjusted accordingly. This works by continously sampling the full range until the energy lies in the selected range, then the ratio of accepted / total attempts is used to bias the simulation time. This is not the optimal approach but it is easy enough to implement (an especially to maintain) and it shouldn't be a problem since sampling the histogram is much faster than doing the radiation transport of an event (unless the energy range is extremely narrow, then this may become a bottleneck).