code: def    fvga(a, i, g, n):
      
      return a * ((((1 + i) ** n) - (((1 + g) ** n)))/(i - g))
docs: This function is for the future value of an annuity
      with growth rate.  It is the future value of a growing
      stream of periodic investments.
      a = Periodic Investment (1000)
      i = interest rate as decimal (.0675)
      g = the growth rate (.05)
      n = the number of compound periods (20)
      Example: fv(1000, .0675, .05, 20)