Add percent support, bump pint version

[kroenlein]

GEMD Python PR

Description

This PR updates the units file to be consistent with the Pint 0.23 release, which includes support for percent & ppm. This also requires bumping Pint minimum version to 0.21 in order to support parsing the % symbol. This does not impact the minimum supported version of Python.

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