Test assessed ranges and metric calculations

Pull request

Please confirm that this pull request has done the following:

• Test pyrcmip.database
• Tests added
• Documentation added (where applicable)
• Example added (either to an existing notebook or as a new notebook, where applicable)
• Description in CHANGELOG.rst added
• pin scmdata >= 0.7.0

Adding to CHANGELOG.rst

Please add a single line in the changelog notes similar to one of the following:

- (`!XX <https://gitlab.com/rcmip/pyrcmip/merge_requests/XX>`_) Added feature which does something
- (`!XX <https://gitlab.com/rcmip/pyrcmip/merge_requests/XX>`_) Fixed bug identified in (`#YY <https://gitlab.com/rcmip/pyrcmip/issues/YY>`_)