fix(growth): alias z-score and use if

lib/growth/calc/centile.ex

@@ -9,17 +9,17 @@ defmodule Growth.Calc.Centile do
     Enum.map(measures, &apply(Growth.Calc.Centile, :compute, &1))
   """
 
+  alias Growth.Calc.ZScore
+
   # TODO: (jpd) add documentation and typespecs
 
   def compute(y, l, m, s) do
-    zscore = Growth.Calc.ZScore.raw(y, l, m, s)
+    zscore = ZScore.raw(y, l, m, s)
 
-    cond do
-      -3 <= zscore and zscore <= 3 ->
-        m * :math.pow(1 + l * s * zscore, 1 / l)
-
-      true ->
-        :na
+    if -3 <= zscore and zscore <= 3 do
+      m * :math.pow(1 + l * s * zscore, 1 / l)
+    else
+      :na
     end
   end
 end