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.

There are two changes:

• This implementation returns a dictionary instead of a Series.
• In accordance with the correspondence between q nulls and Python nulls established in the .pd conversion, the empty string is regarded as a null value for columns of type symbol.

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)

[nipsn]

Overall OK, but there are some adjustments that I think would need to be made before merging.

Also, some interesting flags have been raised in regards to mixed value columns that will need addressing at some point.

[nipsn]

Looks good. Your solution to type checking on columns looks more robust than what I proposed originally.

[nipsn]

Just spoke with the PyKX team. They told me that for this specific case, they would expect the behavior to be closer to what in q would be count distinct.

I think that as it stands right now the implementation follows these lines pretty closely. However, I think that in case we had a mixed type column we should no longer raise a NotImplementedError, so it closely resembles the q behavior. If it fails in q, it should fail in this version as well. We should also change the unit test that monitored this error to instead expect a QError.

TLDR: If we have several nulls of different types on a single column, they should all count as distinct values.