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

This PR adds a standard deviation estimator using MAD, and two simple functions to convert between Gaussian FWHM and sigma.  These calculations were often repeated in `photutils`.

CC: @cdeil, @astrofrog 
@larrybradley - it seems the small comment that this should apply to any dimension data array got missed. Could you still fix that?