Analytic functions with blackbody

See #3077 
@mhvk , I removed overflow catching and I am getting the result below. Based on your comment in #1480 about overflow, this is not the right behavior, right? Here, you say Numpy is already doing the right thing. I am a bit confused.
```
>>> planck(10 * u.m, 1.e10, flux_unit=FNU)
<Quantity 2.7612975999998015e-12 erg / (cm2 Hz s sr)>
```