minimum headerlist in config yamls

[tmerlis]

Is your feature request related to a problem? Please describe.

In building an X-SHiELD catalog, there were several headerlist entries that I wanted to eliminate since most of them were going to be NAs.

What works with the current code is to eliminate some of the headers, leaving many that are NAs:

I could eliminated some unused headers, but if I eliminating frequency, realm, table_id, chunk_freq, causes a problem when col.search is called.

This seemed to be the minimum viable headerlist

headerlist: ["source_id", "experiment_id",
"frequency", "realm", "table_id",
"member_id", "grid_label", "variable_id",
"time_range", "chunk_freq","platform","path"]

If I eliminated frequency, realm, table_id, chunk_freq, I would get a .json and .csv, so it looked like a successful creation of a data catolog. However, an error would happen later when I tried to use the resulting json and col.search was used: there was an error even if the search was only for entries like experiment_id that did have data.

Describe the solution you'd like
To be able to do col.search on a catalog without these entries.

Describe alternatives you've considered
The status quo is fine. Perhaps a minimal intervention is to throw an error at the outset: if the catalog is unusable without realm even if it's just 'NA', have a fatal error as soon as the headerlist is read.

Thank you,
Tim