Added units submodule for working with units and quantities defined in ontologies

[jesper-friis]

Description

Added a new units submodule for working with units and quantities defined in ontologies.

The module provides support to populate a Pint units definition file from EMMO. It also subclasses pint.UnitRegistry, pint.Uni, and pint.Quantity, enhancing them with knowledge from the ontology.

Currently it only supports EMMO as source. But it utilises the references to QUDT and OM such that it is possible to work with IRIs to these ontologies as well.

The PR includes tests and documentation.

Other fixes:

• Silence warning from doctest in literal.py
• More informative error message from Triplestore.value()
• Corrected type annotation of Triplestore.restrictions()
• Made AttrDict pickleable
• Run mkdocs and other tools on GitHub with Python 3.11, since it Python 3.9 doesn't understand TypeAlias

Follow-up tasks

• Allow to update the unit registry from domain or application ontologies.
• Represent quantities the same way as units. The Quantity subclass should know about the ontological IRI corresponding to a Pint Quantity.
• Given IRI to a quantity in the KB, represent it as a Quantity object.
• Store a Quantity object to KB.
• Add physical constants from the ontology.

Type of change

• Bug fix and code cleanup
• New feature
• Documentation update
• Testing

Checklist for the reviewer

This checklist should be used as a help for the reviewer.

• Is the change limited to one issue?
• Does this PR close the issue?
• Is the code easy to read and understand?
• Do all new feature have an accompanying new test?
• Has the documentation been updated as necessary?
• Is the code properly tested?

[codecov]

Codecov Report

Attention: Patch coverage is 83.25991% with 38 lines in your changes missing coverage. Please review.

Project coverage is 77.37%. Comparing base (4fc872f) to head (b4cc37a).

Files with missing lines	Patch %	Lines
tripper/units/units.py	82.80%	38 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #324      +/-   ##
==========================================
+ Coverage   76.83%   77.37%   +0.53%     
==========================================
  Files          22       24       +2     
  Lines        2491     2718     +227     
==========================================
+ Hits         1914     2103     +189     
- Misses        577      615      +38     

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