deltaFunction:

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   The delta function is the non-uniform distributions used by the nonUniform
   mutations.  This function returns a change based on the current gen, the
   max gen and the amount of possible deviation.

   Since version 9.
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   Form:
   [change] = deltaFunction(ct,mt,y,b)
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   ------
   Inputs
   ------  
   ct         current generation
   mt         maximum generation
   y          maximum amount of change, i.e. distance from parameter 
              value to bounds
   b          shape parameter

   -------
   Outputs
   -------
   change

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