Show valid field names

[janscience]

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.

[janscience]

Nice would be that gca-filter -l list all available field names, and .gca-filter -l <filed-name> list possible values (extracted from the json file)