Add mad_std function
Add fwhm_to_sigma and sigma_to_fwhm functions
Implement @embray's comments
Prepend "gaussian_" to FWHM <-> sigma conversion functions
Implement other comments
Define module-level constants for Gaussian FWHM <-> sigma conversions
Updated CHANGES.rst for functonality added in #3208.
Implement @mhvk's comments [skip ci]
Merge pull request #3208 from larrybradley/stats

Add mad_std() and constants to convert Gaussian FWHM <-> sigma