fix(growth): use centile and percentile correctly

1. The calculation made in `Grownth.Calc.Centile.compute/4` converts a
   given z-score into the expected measurement value.
   This can be used to get specific values for key z-scores, such as,
   -3, -2, 2 and 3.
2. To calculate the percentile of a given z-score the system uses the
   cumulative distribution function.

lib/growth/calc/percentile.ex

@@ -0,0 +1,30 @@
+defmodule Growth.Calc.Percentile do
+  @moduledoc """
+  Convert the z-score of a given measurement into a cumulative percentile.
+
+  This calculation is described in the [cumulative distribution function][https://en.wikipedia.org/wiki/Error_function#Cumulative_distribution_function].
+
+  ## Examples
+
+    iex> z_scores = [-1.0, 0.0, 1.0]
+    iex> Enum.map(z_scores, &apply(Growth.Calc.Percentile, :compute, :&1))
+    [0.15865525393145707, 0.5, 0.8413447460685429]
+
+  """
+
+  @doc """
+  Convert the z-score of a given measurement into a percentile representation, ranging from 0 to 1.
+
+  ## Examples
+
+    iex> Growth.Calc.Percentile.compute(2.0)
+    0.9772498680518208
+    iex> Growth.Calc.Percentile.compute(-2.0)
+    0.02275013194817921
+
+  """
+  @spec compute(number()) :: number()
+  def compute(z_score) do
+    0.5 * (:math.erf(z_score / :math.sqrt(2)) + 1)
+  end
+end