wip(growth): calculate key metrics for growth

1. calculate age in days, weeks, and months
2. calculate body mass index for metric and english unities
3. calculate z-score for a given measurement and box-cox parameters (l,
   m, s)
4. calculate centile score for a given measurement and box-cox
   parameters

lib/growth/calc/centile.ex

@@ -0,0 +1,25 @@
+defmodule Growth.Calc.Centile do
+  @moduledoc """
+    measures =
+      [
+        [30, -1.7862, 16.9392, 0.1107],
+        [14, -1.3592, 20.4951, 0.12579],
+        [19, -1.6318, 16.049, 0.10038]
+      ]
+    Enum.map(measures, &apply(Growth.Calc.Centile, :compute, &1))
+  """
+
+  # TODO: (jpd) add documentation and typespecs
+
+  def compute(y, l, m, s) do
+    zscore = Growth.Calc.ZScore.raw(y, l, m, s)
+
+    cond do
+      -3 <= zscore and zscore <= 3 ->
+        m * :math.pow(1 + l * s * zscore, 1 / l)
+
+      true ->
+        :na
+    end
+  end
+end