PR: Signature update of spectral to XYZ definitions in the context of absolute colorimetry computations.

[tjdcs]

Summary

k is passed by position in several functions, so the order of arguments matters. Bug fix.

in discrete integration, you also need to multiply by ∆λ

adds unit tests

Preflight

Code Style and Quality

• [x] Unit tests have been implemented and passed.
• [x] Mypy static checking has been run and passed.
• [x] 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

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