Using a much smaller minimum weight value in Pythia8

We seem to have an old check for a "very small" weight, where that very small value is fairly arbitrary. This seems to now be actually affecting some BSM samples that are being generated.

Changing the check to use float minimum, because I fear these weights are cast into floats at some point in the analysis chain even if they aren't right here in this class. Also printing a warning in case the weight is between double and float mins, to at least tell the user what's happening a bit more clearly.