HorneExtract option to use reference star kernel

Currently HorneExtract assumes a Gaussian kernel. Option to import a custom kernel, perhaps in the form of a reference star 2D spectrum would be useful in the case where the 2D spectrum is too noisy to generate an accurate self-kernel.

Start by locating the HorneExtract implementation and the code that assumes a Gaussian kernel. Determine how a custom kernel, including a reference star 2D spectrum, should be supplied and what existing extraction checks or examples cover it. Done means HorneExtract can use the reference-star kernel for noisy 2D spectra, with behavior covered by an appropriate test or example.

Written by the indexing model from the issue text.