Analytic functions with blackbody

See #3077 
To address this and comments on renaming the functions: `planck()` has been renamed to `blackbody_nu()` and just gives F_nu result. `blackbody_lambda()` is added to wrap around `blackbody_nu()` to give F_lambda. I removed the `flux_unit` option. If users want to convert to something other than F_lambda or F_nu, they can always just use `spectral_density()` equivalency.