Powderday only looks for dust in PartType0 with otf extinction off

Currently, powderday assumes that the dust is in `PartType0` if `otf_extinction` is set to `False` for `AREPO` simulations and does not even check if it is in `PartType3` instead. This caused issues on an attempted run to use the MW grain size distribution for a `SMUGGLE` galaxy by setting `otf_extinction` to `False`.