Implement extended sources

[mjlarson]

Looked into this already with the help of Claude.ai. Sounds like it's relatively simple to do this in spherical harmonic space, so we'd need to migrate the get_king_bl function from the TemplateSmearedKingPDF class to some place for calling in gaussian extended sources. I'm not sure if that should be in the standard KingPDF/InterpolatedKingPDF or if it should go into a separate class. I'm considering the latter to avoid adding too much complication, but it needs more thought...

[mjlarson]

#7 implements this by directly smearing the event positions when we build the histograms prior to fitting, so we don't touch spherical harmonic space at all.