Exclude vars from catalogs in parse_directory

[aulemahal]

Pull Request Checklist:

• pre-commit hooks are installed/active in my local clone ($ pre-commit install)
• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
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What kind of change does this PR introduce?

In the "variable" mapping passed to cvs in parse_directory, mapping a var name to None will exclude that var from the catalog. Meaning it won't appear in the variable column. The file entry is untouched. This is useful when one is reading the variables from the files (using read_from_file). It allows excluding things like time_bnds`.

Another technique would have been passing drop_variables to xr_open_kwargs, but this is not universal as the optimization will use netCDF4 or zarr to open the files when they can.

Does this PR introduce a breaking change?

No.

Other information:

@mccrayc