Is it possible to define this constant in `astropy.constants`?
Here's an analytical formula(to get correct results up to rounding error precision):
```
In [12]: import numpy as np
In [13]: sigma_to_fwhm = np.sqrt(8 * np.log(2))         
In [14]: fwhm_to_sigma = 1 / sigma_to_fwhm
In [15]: sigma_to_fwhm
Out[15]: 2.3548200450309493
In [16]: fwhm_to_sigma
Out[16]: 0.42466090014400953
```
I frequently use this constant in my own analysis code, so having it available in `astropy.constants` would be nice.
I can make a separate pull request if someone tells me where it would fit best.