Avoid a circular import by moving load_tree into kdtree_generator

[kbwestfall]

As titled.

[rcooke-ast]

This looks good to me 👍

I burned many hours attempting to solve the ThAr automatic wavelength calibration problem, and I think this is not the solution. Perhaps there's a better solution that we could reconsider at some point, but I think for now, perhaps we could raise an error if someone attempts to load a kdtree, to say that this is currently not supported?

[kbwestfall]

This looks good to me 👍

I burned many hours attempting to solve the ThAr automatic wavelength calibration problem, and I think this is not the solution. Perhaps there's a better solution that we could reconsider at some point, but I think for now, perhaps we could raise an error if someone attempts to load a kdtree, to say that this is currently not supported?

Thanks for the quick review, @rcooke-ast ! I added an exception if the user tries to use 'holy-grail' with ThAr arcs. Is this what you were thinking?

[rcooke-ast]

Precisely, thanks for implementing that!

[profxj]

Indeed, this is nearly deprecated code, but no harm in keeping
it around for now.