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.
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