Iterate SWIFTGalaxy's efficiently

[kyleaoman]

If we want to iterate over many galaxies that lie in the same top-level cell(s) in the snapshot then creating separate SWIFTGalaxy objects will be inefficient because each time a SWIFTGalaxy is created the same particles will be read from disk, and then all but those corresponding to the halo of interest will be discarded. Instead we would like to do the expensive disk i/o once and then temporarily mask out particles that don't belong to the current galaxy of interest while iterating over galaxies.