I/O: Allow user to add netCDF metadata in YAML control file

This isn't a high priority feature, but shouldn't be too hard to implement.

It might be nice to allow a user to add metadata fields to the netCDF header file via the output yaml file. For example, if a user wants to add contact info for who ran the simulation.

Locate the YAML output configuration and the code that writes the netCDF header, since the issue names no files or tests. Check how output options are parsed and how headers are currently populated. Done means metadata fields supplied in the output YAML appear in the generated netCDF header.

Written by the indexing model from the issue text.