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/bmi.ex

@@ -0,0 +1,29 @@
+defmodule Growth.Calc.BMI do
+  @moduledoc """
+  Calculate body mass index
+  """
+
+  @inch_to_centimeter 2.54
+  @pound_to_kilogram 0.453592
+
+  @doc """
+  Calculate the body mass index for a given weight and height.
+
+  Measurements taken in english unit (pounds and inches) are converted to their metric equivalents (kilograms and centimeters).
+  """
+  @spec calculate(:english | :metric, number, number) :: number
+
+  def calculate(:english, weight, height) do
+    metric_weight = pound_to_kilogram(weight)
+    metric_height = inches_to_centimeters(height)
+    calculate(:metric, metric_weight, metric_height)
+  end
+
+  def calculate(:metric, weight, height) do
+    weight / :math.pow(height / 100.0, 2)
+  end
+
+  defp inches_to_centimeters(height), do: height * @inch_to_centimeter
+
+  defp pound_to_kilogram(weight), do: weight * @pound_to_kilogram
+end