GAUSSIAN

       Compute the 1-d Gaussian function and optionally the derivative

       Compute the 1-D Gaussian function and optionally the derivative
       at an array of points.

       y = gaussian( xi, parms,[ pderiv ])

       xi = array, independent variable of Gaussian function.
       parms = parameters of Gaussian, 2, 3 or 4 element array:
               parms[0] = maximum value (factor) of Gaussian,
               parms[1] = mean value (center) of Gaussian,
               parms[2] = standard deviation (sigma) of Gaussian.
               (if parms has only 2 elements then sigma taken from previous
               call to gaussian(), which is stored in a  common block).
               parms[3] = optional, constant offset added to Gaussian.

       y -  Function returns array of Gaussian evaluated at xi.    Values will
            be floating pt. (even if xi is double) unless the /DOUBLE keyword
            is set.

       /DOUBLE - set this keyword to return double precision for both
             the function values and (optionally) the partial derivatives.

       pderiv = [N,3] or [N,4] output array of partial derivatives,
               computed only if parameter is present in call.
               pderiv[*,i] = partial derivative at all xi absisca values
               with respect to parms[i], i=0,1,2,[3].

       Evaulate a Gaussian centered at x=0, with sigma=1, and a peak value
       of 10 at the points 0.5 and 1.5.   Also compute the derivative
       IDL> f = gaussian( [0.5,1.5], [10,0,1], DERIV )
       ==> f= [8.825,3.25].   DERIV will be a 2 x 3 array containing the
       numerical derivative at the two points with respect to the 3 parameters.

       None

       Written, Frank Varosi NASA/GSFC 1992.
       Converted to IDL V5.0   W. Landsman   September 1997
       Use machar() for machine precision, added /DOUBLE keyword,
       add optional constant 4th parameter    W. Landsman   November 2001

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