Partitioner - to fix error if partitions have size 1

[MargaretDuff]

Describe your changes

Fixed issue and added a new unit-test

Describe any testing you have performed

Now works:

Link relevant issues

closes #1464

Checklist when you are ready to request a review

• I have performed a self-review of my code
• [X ] I have added docstrings in line with the guidance in the developer guide
• [X ] I have implemented unit tests that cover any new or modified functionality
• [ X] CHANGELOG.md has been updated with any functionality change
• [X ] Request review from all relevant developers
• [X ] Change pull request label to 'Waiting for review'

Contribution Notes

Please read and adhere to the developer guide and local patterns and conventions.

• [ X] The content of this Pull Request (the Contribution) is intentionally submitted for inclusion in CIL (the Work) under the terms and conditions of the Apache-2.0 License.
• [ X] I confirm that the contribution does not violate any intellectual property rights of third parties

[gfardell]

Add some comments explaining the new unit tests, but otherwise it looks good!

[gfardell]

If you write "closes #1464" or "fixes #1464" in the PR linked issues it'll close the issue automatically when the PR is merged.

[paskino]

I added just some pedantic suggestions.

Please write "closes #1464" in the text.