grouping and reduction operation on subhalos

[dnelson86]

We need support, as now for groups, but for subhalos.

For example, a command like:

mass = ds.grouped("Masses", parttype="PartType0").sum().evaluate(compute=True)

except reducing over subhalos. (Each subhalo is a contiguous slice of the particle arrays, just like a group).

[dnelson86]

(More abstractly, a catalog can have more than one set of "groups")