Adds isnull, isna, notnull, notna, idxmax, idxmin, kurt and sem functions

[nipsn]

Feature

• Please insert link to associated issue here: /issues/23

What does this change introduce?

An implementation of the isnull, isna, notnull and notna functions:

• https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.isnull.html#pandas.DataFrame.isnull
• https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.isna.html#pandas.DataFrame.isna
• https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.notnull.html#pandas.DataFrame.notnull
• https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.notna.html#pandas.DataFrame.notna

An implementation of the idxmax and idxmin functions:

• https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.idxmax.html
• https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.idxmin.html

An implementation of kurt: https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.kurt.html

An implementation of sem: https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.sem.html

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

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

Testing

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

Documentation

• [x] 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
• [x] Have you checked your changes with a spell checker? (US English)

[nipsn]

@cmccarthy1 @rianoc-kx Please review our contributions and feel free to give any kind of feedback. Thanks in advance.

[cmccarthy1]

@nipsn taking a look at this now, will see if we can get it merged for our 2.4 release next week but suspect it may be 2.5 for these additions