More than 1 galaxy type for API

[JoanneBogart]

There is no reason to create more than one galaxy type per invocation of create_sc.py - in fact normally catalogs for only one object type altogether are created at a time. But the API should be able to read catalogs for more than one type in the same session.
In order of desirability

1. Verify this works properly now
2. Update code to make it work
3. Constrain users to simulate only one galaxy type per usage of skyCatalogs API

[JoanneBogart]

One thing standing in the way of reading both diffsky and cosmodc2 catalogs is that, by default, they use the same file name templates: galaxy_.parquet for main file and galaxy_flux_.parquet (and are expected to be in the same directory as the config file). With current code this can probably (awkwardly) be overridden at creation time and again when reading, but it would be better to just use distinct names.