Automate the specification of some user configuration variables

#109 makes some request keys configurable via CMEW. In time, some of these could be deduced from other configuration variables.

For example:
- atmos_timestep: potentially could be determined from model resolution if we have this?
- institution_id: derive from site?

There may be others - all configuration variables should be reviewed.

Start with issue #109 and the existing CMEW configuration handling for request keys. Review all configuration variables, including atmos_timestep and institution_id, and identify which can be derived from others. Done means the derivable variables and their source configuration are agreed and the automation scope is defined; no files or tests are named in the issue.

Written by the indexing model from the issue text.