Update colour.SpectralDistribution.interpolate method to use continuous-based integration.

[KelSolaar]

Summary

Update colour.SpectralDistribution.interpolate method to use continuous-based integration.

This provides support for fractional spectral-power distribution interpolation and numerical differences are to be expected in such cases.

References #1325.

Preflight

Code Style and Quality

• Unit tests have been implemented and passed.
• Pyright 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: 97.483% (+0.005%) from 97.478%
when pulling c431ecb on feature/continuous_interpolation
into 4349fb7 on develop.