Add units setter in framework, update readers to add units

[hrobarts]

Changes

Formalise use of units in CIL

• Use units.py to store accepted units and conversions (currently in Phase contribution #1737)
• Add a units setter in framework
• Set units in readers where the information is available

Some considerations

• Need some thought about where unit conversions happen, we don't want conversion to happen repeatedly between functions as this could add computational cost in our algorithms
• Conversions could only occur at user input or output
• Allow no units/ custom units as a valid option

Testing you performed

Please add any demo scripts to https://github.com/TomographicImaging/CIL-Demos/tree/main/misc

Related issues/links

Checklist

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

Contribution Notes

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

• 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
• I confirm that the contribution does not violate any intellectual property rights of third parties