Test clearing catalogue with #clean

[steve9164]

What this PR does

Fixes #6701.

#clean will now reset terria.catalog.group. Not to be merged before adding garbage collection of unnecessary models.

Test me

Go to http://ci.terria.io/improve-clean/, then add #clean to the URL.

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.