subtypes of particle types

[danieljprice]

with multigrain dust output, we need to be able to define particle "sub-types" as child types of a single generic type. Otherwise we end up with potentially hundreds of particle types which make splash unusable.

The idea is to create a single "family", so e.g. type 2 particles are still "dust", but within the dust type there are a bunch of subtypes. Of course this is just a fancy way of accessing particle types 7->n, but means high level operations (like turning all dust on/off) can be done easily.

Could also apply to star particles, e.g. bulge/disc/halo could all be subsets of the "star" particle type.

Implementing this is a bit tricky, but urgently needed