Math operations and binning should treat `.model`(s) in `RainbowWithModel` the same as `flux`.

[zkbt]

Right now, if we bin a RainbowWithModel, the flux gets uncertainty weighting, but everything else gets uniform weighting. Likewise, if we do math on a Rainbow, the flux and uncertainty?!?!) are affected by the calculation, but nothing else.

For making it easier to propagate models through multiple Rainbow actions, we should change the RainbowWithModel to:

• [x] use uncertainty-weighting for model (and all other 2D quantities with model in their names?) when doing .bin()
• [x] treat model (and all other 2D quantities with model in their names?) like flux when doing math

[zkbt]

Oh fudge! This made me check; right now normalize() affects uncertainty, but none of the basic math operations do. That's a bug. When we do rainbow*2, both its flux and its uncertainty should be multiplied by 2.

[zkbt]

Interestingly, it turned out that inverse-variance uncertainty weighting was actually happening already. All fluxlike quantities get uncertainty weighting. None of the wavelike or timelike ones get uncertainty weighting because it would require too many choices and would produce uneven time/wavelength grids that cause more trouble than they'd be worth.