Show valid field names

You could add a '-l' argument to gca-filter and gca-select scripts that simply lists all valid field names and if appropriate valid values. Right now one has to dig into the code to figure that out.

The Abstract class field names differ from the ones in the json file. That does not help, either. Maybe this can be unified.

Start with the gca-filter and gca-select scripts, then inspect the Abstract class and the JSON field names they expose. Determine how the -l option should list valid fields and values, and whether the Abstract and JSON names should be unified. Done means the scripts clearly report the supported names and values without requiring users to inspect the code.

Written by the indexing model from the issue text.