FUNCTION fwhm2sigma, fwhm

; Description: This function converts a Gaussian FWHM to a Gaussian sigma.
;
; Input Parameters:
;
;   fwhm - FLOAT/DOUBLE - The Gaussian FWHM. This parameter must be non-negative.
;
; Return Value:
;
;   The function returns the sigma of the Gaussian as the same variable type and
;   with the same units as the input parameter "fwhm". If "fwhm" is negative, or 
;   the function fails, then it returns a value of "-1.0"
;
; Author: Dan Bramich (dmb@ing.iac.es)
;
; History:
;
;   15/07/2008 - Module created (dmb)

;Check that "fwhm" is a non-negative number of the correct type

;Convert the Gaussian FWHM to a Gaussian sigma