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/age.ex

@@ -0,0 +1,40 @@
+defmodule Growth.Calc.Age do
+  @moduledoc """
+  Calculate the age in months.
+  """
+
+  # NOTE: (jpd): based on [WHO instructions][0]
+  # [0]: https://cdn.who.int/media/docs/default-source/child-growth/child-growth-standards/indicators/instructions-en.pdf
+  @day_in_month 30.4375
+
+  @day_in_week 7.0
+
+  @doc """
+  Calculate the age with the precision of day, or week, or month. For age in weeks or months, considers completed ones, removing decimal value.
+
+  If only date of birth is available, assumes that measurement date is today.
+  """
+  @spec calculate(:day | :week | :month, Date.t(), Date.t()) :: pos_integer()
+
+  def calculate(precision, date_of_birth) do
+    calculate(precision, date_of_birth, Date.utc_today())
+  end
+
+  def calculate(:month, date_of_birth, date_of_measurement) do
+    :day
+    |> calculate(date_of_birth, date_of_measurement)
+    |> Kernel./(@day_in_month)
+    |> floor()
+  end
+
+  def calculate(:week, date_of_birth, date_of_measurement) do
+    :day
+    |> calculate(date_of_birth, date_of_measurement)
+    |> Kernel./(@day_in_week)
+    |> floor()
+  end
+
+  def calculate(:day, date_of_birth, date_of_measurement) do
+    Date.diff(date_of_measurement, date_of_birth)
+  end
+end