Hotfix terra handling

[znicholls]

Pull request

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable) (N/A)
• Example added (either to an existing notebook or as a new notebook, where applicable) (N/A)
• Description in CHANGELOG.rst added

Adding to CHANGELOG.rst

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

- (`#XX <https://github.com/openscm-project/openscm-units/pull/XX>`_) Added feature which does something
- (`#XX <https://github.com/openscm-project/openscm-units/pull/XX>`_) Fixed bug identified in (`#YY <https://github.com/openscm-project/openscm-units/issues/YY>`_)