POSITIVITY

       Map an image uniquely and smoothly into all positive values.

       Take unconstrained x (usually an image), and map it uniquely and
       smoothly into positive values.   Negative values of x get mapped to
       interval ( 0, sqrt( epsilon )/2 ], positive values go to
       ( sqrt( epsilon )/2, oo ) with deriv approaching 1.  Derivative is
       always 1/2 at x=0.   Derivative is used by the MRL deconvolution
       algorithm.

       result = POSITIVITY( x, [ /DERIVATIVE, EPSILON = )

       x - input array, unconstrained

       result =  output array = ((x + sqrt(x^2 + epsilon))/2
               if the /DERIV keyword is set then instead the derivative of
               the above expression with respect to X is returned

       DERIV -  if this keyword set, then the derivative of the positivity
               mapping is returned, rather than the mapping itself
       EPSILON - real scalar specifying the interval into which to map
               negative values.    If EPSILON EQ 0 then the mapping reduces to
               positive truncation.   If EPSILON LT then the mapping reduces to
               an identity (no change).  Default is EPSILON = 1e-9

        F.Varosi NASA/GSFC 1992, as suggested by R.Pina UCSD.
       Converted to IDL V5.0   W. Landsman   September 1997

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