Review and correct units in data variables file

The data science team need a canonical source of units for the outputs of the model and the best place for that is through the `data_variables.toml` file. That's currently very out of date and inaccurate. Once it is fixed:

1. Our data variables table will be correct.
2. We can use the same data to populate the `unit` attributes on output NetCDF files.

Start by locating the `data_variables.toml` file and reviewing how its units correspond to model outputs. Confirm the correct units with the relevant data science or model definitions, then ensure the table is accurate enough to provide `unit` attributes on output NetCDF files.

Written by the indexing model from the issue text.