Analytic functions with blackbody

See #3077 
Haha, you are too observant. I was hoping no one would catch that! ;) Yes, you are right. The original function calculates in B_lambda (PHOTLAM, to be exact). When I modified it, I found it to be more robust to calculate in B_nu, and then convert to PHOTLAM. But obviously, I took out the final conversion here, so yes, I will update the formula.