Investigate whether the native option could work for model outputs

An alternative option to #20 is to consider whether the built-in native option in ESMValTool could work for model outputs.

There is a potential issue in that even using the native option could cause issues, since it will only work in cases where one STASH code corresponds to one CMIP variables.

**Details**

- Estimate: Spend up to half a day on this

Start by reviewing issue #20 and ESMValTool's built-in native option for model outputs. Check whether the option supports cases where one STASH code maps to one CMIP variable, then record whether it is suitable for CMEW and any limitations found.

Written by the indexing model from the issue text.