Initial multiband implementation

[tbmiller-astro]

Initial implementation of joint modeling across multiple bands

The FitMultiBand[X] classes take a list of FitSingle or FitMulti classes as input along with an array of the wavelengths at each band. The parameters specified by linked_params are jointly fit across all bands while all others are fit independently. There are currently two implementations: The first, following previous implementations, "linking" these parameters using polynomials and the other using B-Splines. These implementations are basic but should be fully functional.

Things to-do before merge-able

• Finalize UI and how the classes are initialized
• Finalize interface with results class
• Finalize documentation and add tutorial
• Add tests
• Currently necessitates pixel scale and zero points are the same across all images, but should be simple enough to add the ability to vary this (just a multiplicative factor)
• Maybe more?

[tbmiller-astro]

Okay I am pretty happy with where this is now.

The only major thing we could add is difference in pixel scale and flux scaling. I don't have a huge need for this now but it could be useful for some.