Add collapse operator with corresponding yaml file and changes

[Sylviabohnenstengel]

Review requested for when you are available next week.

first draft of aggregator dealing with min, max, stdev, etc.
The percentile aggregator is included with further line in yaml file and if condition in aggregator using the **kwargs argument. Interested in solution that looses the if condition when you review the code.

Fixes #99

[Sylviabohnenstengel]

renaming the aggregate operator to collapse operator as it is doing a collaps using min, max, percentile functions.

[Sylviabohnenstengel]

https://github.com/MetOffice/CSET-workflow/pull/86

[jfrost-mo]

At a quick glance the operator looks reasonable. I'll do a more detailed review later today.

[jfrost-mo]

Generally looks good to me. Some minor changes suggested around the code. I did also wonder whether we wanted to generalise the operator around how many dimensions it can collapse at once. See my comment on the collapse_cube_2dim function.

[jfrost-mo]

To get the tests passing this branch needs rebasing on main. You can do that as follows:

git pull
git rebase origin/main aggregator_operator
git push --force

[Sylviabohnenstengel]

Hi, I have gone through your suggestions and pushed the changes. Could you take a look please? I had already started on the aggregate operator so hope none of the aggregator related code has made it into this push.

[jfrost-mo]

I've reformatted the code, removed the unnecessary collapse_2dim operator, and gotten the tests passing.

The 1 dim function already actually handled collapsing over multiple coordinates, so I just updated its docstring.

[github-actions]

[jfrost-mo]

Only thing left to do is to add a test for the PERCENTILE method, and it should be ready to merge.

[jfrost-mo]

I'm happy with this now. @Sylviabohnenstengel if you are happy, go ahead and merge in this PR.

[Sylviabohnenstengel]

Do we still use the test_data/*yaml files?