use generic broadband AOD in Bird model instead of only Bird-Hulstrom approximation

[mikofski]

Probem

1. Currently the Bird clear sky model only takes arguments for aod380, aod500 and uses the Bird-Hulstrom approximation for broadband AOD.
2. There are other broadband approximations that might be better such as Molineaux approximation which is what Ineichen suggests for simplified Solis.
3. The signature of the arguments for the Bird model are different from the Simplified Solis, would be better if they were consistent, so instead of asking for two AOD arguments, the Bird model should only ask for aod_bb. Huh, should we update the Solis also, since it actually asks for aod700?

Solution

1. Just replace the two AOD arguments in Bird with a single broadband AOD argument
2. replace the calculation of bird_hulstrom broadband approximation with the same argument from (1)
3. replace the variable occurances everywhere for bird_hulstrom with aod_bb.
4. update the documentation
5. maybe consider changing aod700 argument/variable in solis to aod_bb

related to #276, #269 and #278

[wholmgren]

I support @mikofski's proposed changes to the bird clear sky model implementation. Let's make sure the clear sky model's documentation makes reference to the bird_hulstrom function.

I think we should keep using aod700 in simplified solis, at least for now.