Investigation: How to add a bespoke project to ESMValTool

It will be necessary to update the `project`, `dataset`, `start_year` and `end_year` (and `ensemble`?) keys in the `datasets` dictionary in the ESMValTool recipe, see #18 for more information and see the [radiation budget recipe](https://github.com/ESMValGroup/ESMValTool/blob/main/esmvaltool/recipes/recipe_radiation_budget.yml) for an example.

The radiation budget recipe can currently be run as follows:

```
module load scitools/community/esmvaltool/2.6.0
esmvaltool run recipe_radiation_budget.yaml
```

(see `esmvaltool run --help` for information about running installed and local recipes)

This investigation will involve answering the following questions:

- Does the radiation budget recipe complete without errors if the `project` in a copy of the radiation budget recipe is updated to an arbitrary name, e.g. `CMEW`?
- Does the arbitrary name need a corresponding entry in the `rootpath` and `drs` sections in the [ESMValTool user configuration file](https://github.com/ESMValGroup/ESMValTool/blob/main/config-user-example.yml#L227)?
- Does the radiation budget recipe complete without errors if the `dataset` in a copy of the radiation budget recipe is updated to a model run ID, e.g. `u-ab123`?
- Does the radiation budget recipe complete without errors if the `ensemble` is also updated?
- If this doesn't work, what would be needed to get this to work? For example, is there a pre-defined list of `project` hard-coded in ESMValTool? If so, can this list be updated at run time?
- Is there any ESMValTool documentation that describes adding a new `project`?

**Details**

- Estimate: Spend no more than 3 days on this investigation.
- Document the answers to the above questions on [wiki: Investigation: How to add a bespoke project to ESMValTool](https://github.com/MetOffice/CMEW/wiki/Investigation:-How-to-add-a-bespoke-project-to-ESMValTool)
- There is a [developer configuration file](https://github.com/ESMValGroup/ESMValCore/blob/main/esmvalcore/config-developer.yml) that may help :)
- [ESMValTool documentation: Automatically populating a recipe with all available datasets](https://docs.esmvaltool.org/projects/esmvalcore/en/latest/recipe/overview.html#automatically-populating-a-recipe-with-all-available-datasets) may also be useful.

Start by copying the radiation budget recipe and running it with the documented esmvaltool command. Compare the recipe with config-user-example.yml and esmvalcore/config-developer.yml while testing the project, dataset, start_year, end_year and ensemble changes described. Record the answers, requirements and relevant documentation on the linked CMEW wiki page.

Written by the indexing model from the issue text.