Performance request: make name optional

[steven-murray]

When reading in a large sky model, for example the GSM in healpix format, reading in the name of each source is a huge bottleneck: for example it takes ~1 minute to read in an Nside=1024 catalog, of which >90% is spent decoding the string names. For such a catalog, the component names are kind of useless -- it's easier just to think of them by their index. Can we make name optional?