FUNCTION fwhm2sigma, fwhm

; Description: This function converts a Gaussian FWHM to a Gaussian sigma. The input
;              parameter "fwhm" may be a scalar, vector or array.
;
; Input Parameters:
;
;   fwhm - FLOAT/DOUBLE SCALAR/VECTOR/ARRAY - 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". Where "fwhm" is negative, 
;   a value of "-1.0" is returned. If the input parameter "fwhm" is a scalar, then 
;   the return value will be a scalar, and if the input parameter "fwhm" 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 "fwhm" is of the correct number type

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

;Return the Gaussian sigma value(s)