Exozodi wavelength scaling is wrong

[dsavransky]

Bug report courtesy of Chris Stark:

I looked at the ExoSIMS documentation re how it calculates exozodi surface brightness (https://exosims.readthedocs.io/en/latest/concepts.html#zodiacal-and-exozodiacal-light). It looks like it is calculating using the same method as AYO, except for the wavelength dependence. In the documentation it states that ExoSIMS first calculates the V band surface brightness, then adjusts it to the wavelength of interest by using the local zodi's color. I've attached the equation in the documentation that shows this.

This is not correct. To first order local zodi is a gray scatterer. Attached is a plot where I have taken the zodi's wavelength dependence (from the ExoSIMS documentation site) and overlaid the Sun's spectrum in the same units and scale. As you can see, the zodi pretty much follows the Sun's spectrum until thermal emission kicks in.

So assuming exozodi is also a gray scatterer (honestly, there's no other good choice at this point), this means that the attached equation should really be

I_exozodi(r,lambda_0) = I^V_exozodi(r) * (F_star(lambda_0)/F_star(550 nm))

i.e., the exozodi should scale with the target star's color, not the local zodi's color.

[dsavransky]

Scope creep: SimulatedUniverse (prototype) the fEZ attribute becomes fEZ_1AU (name TBD) and is now keyed by mode (so dict of arrays instead of array). This is computed for all stars in the TargetList at init and cached. Add new method to SimulatedUniverse that is current_fEZ (name TBD) which uses the internal planet orbital radius state (self.d) to scale the 1 AU value to the current correct value in the requested band (so takes mode as input).