config files: load existing tables from files

[ntessore]

It would be useful to have syntax to load tables from existing files (not necessarily produced by SkyPy, so with arbitrary extension names for FITS or paths for HDF5). I think loading can somehow be achieved already using the .init table constructor, so we should explore. And if that method is not very comfortable to use, we should investigate a more specific table loader syntax. In either case, an example should be provided.