built in operator: softmax

    description:
        softmax sp
        implement the softmax function
        
    algorithm:
        see: https://en.wikipedia.org/wiki/Softmax_function

    examples:
        -- let's consider a Gaussian:
        Gaussian[0.7] |age: 40>
            0.017|age: 36> + 0.047|age: 37> + 0.13|age: 38> + 0.36|age: 39> + |age: 40> + 0.36|age: 41> + 0.13|age: 42> + 0.047|age: 43> + 0.017|age: 44>
            
        bar-chart[50] Gaussian[0.7] |age: 40>
            ----------
            age: 36 :
            age: 37 : ||
            age: 38 : ||||||
            age: 39 : ||||||||||||||||||
            age: 40 : ||||||||||||||||||||||||||||||||||||||||||||||||||
            age: 41 : ||||||||||||||||||
            age: 42 : ||||||
            age: 43 : ||
            age: 44 :
            ----------
        
        -- now softmax it:
        softmax Gaussian[0.7] |age: 40>
            0.085|age: 36> + 0.087|age: 37> + 0.095|age: 38> + 0.12|age: 39> + 0.227|age: 40> + 0.12|age: 41> + 0.095|age: 42> + 0.087|age: 43> + 0.085|age: 44>

        bar-chart[50] softmax Gaussian[0.7] |age: 40>
            ----------
            age: 36 : ||||||||||||||||||
            age: 37 : |||||||||||||||||||
            age: 38 : ||||||||||||||||||||
            age: 39 : ||||||||||||||||||||||||||
            age: 40 : ||||||||||||||||||||||||||||||||||||||||||||||||||
            age: 41 : ||||||||||||||||||||||||||
            age: 42 : ||||||||||||||||||||
            age: 43 : |||||||||||||||||||
            age: 44 : ||||||||||||||||||
            ----------

        softmax (|a> + 2|b> + 3|c> + 4|d> + 5|e> + 6|f>)
            0.004|a> + 0.012|b> + 0.032|c> + 0.086|d> + 0.233|e> + 0.634|f>

    see also:
        rescale, srescale, normalize, snormalize