FUNCTION sigma2fwhm, sigma

; Description: This function converts a Gaussian sigma to a Gaussian FWHM.
;
; Input Parameters:
;
;   sigma - FLOAT/DOUBLE - The Gaussian sigma. This parameter must be non-negative.
;
; Return Value:
;
;   The function returns the FWHM of the Gaussian as the same variable type and
;   with the same units as the input parameter "sigma". If "sigma" 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 "sigma" is a non-negative number of the correct type                  

;Convert the Gaussian sigma to a Gaussian FWHM