PR: Ensure that conversion from spectral to XYZ returns non-percentage values when "k" is not "None".

[KelSolaar]

Summary

This PR ensures that conversion from spectral to XYZ returns non-percentage values when k is not None.

References #1018.

Preflight

Code Style and Quality

• Unit tests have been implemented and passed.
• Mypy static checking has been run and passed.
• Pre-commit hooks have been run and passed.
• [N/A] New transformations have been added to the Automatic Colour Conversion Graph.
• [N/A] New transformations have been exported to the relevant namespaces, e.g. colour, colour.models.

Documentation

• [N/A] New features are documented along with examples if relevant.
• [N/A] The documentation is Sphinx and numpydoc compliant.

[coveralls]

Coverage increased (+3.0e-05%) to 99.836% when pulling 5f706c9 on feature/k_absolute into 4ce59f3 on develop.