Add addtional synonyms for ratio units

[kroenlein]

GEMD Python PR

Description

Follow up for #219 with additional synonyms and definitions.

PR Type:

• Breaking change (fix or feature that would cause existing functionality to change)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Maintenance (non-breaking change to assist developers)

Adherence to team decisions

• I have added tests for 100% coverage
• I have written Numpy-style docstrings for every method and class.
• I have communicated the downstream consequences of the PR to others.
• I have bumped the version in __version__.py

[anoto-moniz]

LGTM