Added implementation of nunique function

[chraberturas]

Feature

• Solves: Feature: Implement nunique from Pandas API #16

What does this change introduce?

An implementation of the nunique function: https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.nunique.html

This is intended to be a 1:1 implementation of the nunique function from pandas.

The only change is that this implementation returns a dictionary instead of a Series.

General

• Has an example been added to demo the new feature?
• Have existing examples been updated or tested?
• Have you added any new Environment Variables/Configuration Options? If yes please tick the boxes below as applicable
  • Addition to reimporter logic within src/pykx/pykx.q and src/pykx/reimporter.py
  • Have updated the src/pykx/util.py logic which is used for environment variable
• If there have been any dependency updates have they been reflected in all files?
  • pyproject.toml
  • docs/getting-started/installing.md
  • conda-recipe/meta.yaml
  • README.md
• If any examples have been updated has it's associated .zip been updated

Code

• Has all temporary code used during development been removed?
• Has all commented out (unused) code been removed?
• Where reasonable have you ensured there is no duplication of existing code?
• If applicable for your use-case have you ensured that the code is performant?

Testing

• Have unit tests been created or existing ones updated to test this new functionality?

Documentation

• Has documentation been added for all public code?
• Has a release note been included for the new feature?
• Has any documentation which would benefit from this feature been updated to use the most up to date functionality?
• If a new class has been added has a documentation stub .md file associated with it been created?
• If any documentation page has been created has it been added to mkdocs.yml
• Have you checked your changes with a spell checker? (US English)

[marcosvm13]

I leave this alternative syntax in case someone finds it useful to understand Christian's incredible composition:

q("(count distinct @) each", res)