FUNCTION sigma2fwhm, sigma

; Description: This function converts a Gaussian sigma to a Gaussian FWHM. The input
;              parameter "sigma" may be a scalar, vector or array.
;
; Input Parameters:
;
;   sigma - FLOAT/DOUBLE SCALAR/VECTOR/ARRAY - 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". Where "sigma" is negative,
;   a value of "-1.0" is returned. If the input parameter "sigma" is a scalar, then
;   the return value will be a scalar, and if the input parameter "sigma" is a vector
;   or an array, then the return value will be a vector or an array, respectively.  
;   If the function fails, then it returns a value of "-1.0".
;
; Author: Dan Bramich (dan.bramich@hotmail.co.uk)
;
; History:
;
;   20/04/2009 - Module expanded to accept vectors and arrays for the input parameter 
;                (dmb).
;   15/07/2008 - Module created (dmb)

;Check that "sigma" is of the correct number type

;Convert the Gaussian sigma to a Gaussian FWHM, setting negative values to "-1.0"

;Return the Gaussian FWHM value(s)