Implement 100% test coverage.

[aaronweeden]

Description

Backport of #27 for the v1.x.y branch.

Type of change:

• Refactoring / documentation update (non-breaking change which does not change functionality)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Release preparation

Checklist:

• CHANGELOG.md has been updated
• xdmod_data/__version__.py has been updated to the next development version
• The milestone is set correctly on the pull request
• The appropriate labels have been added to the pull request
• Running the automated tests (see docs/developing.md) produces no errors
• Updates have been made to the xdmod-notebooks repository as necessary, and the notebooks all run successfully
• The changes in this PR have been ported/backported to other branches as needed