✨ Provide/improve support for units

[mbercx]

It's important that users can easily understand what units the parsed data has.

• Make separate module with chosen units in one place: use consistently throughout package, and use this as SSOT for both code base and documentation.
• Add units with pint: make optional dependency? We have to check how heavy the package is, and how complicated the code would be to make it optional.

Note

If we do support pint, we may have to choose pint~=0.24.4, since v0.25 dropped support for Python 3.9 and 3.10 (!): https://pypi.org/project/Pint/0.25/. This is most likely related to numpy, see this comment