Added Zemel 2015 Down syndrome growth calculator

Calculates Z-scores and percentiles based on the Zemel 2015 study for children ages 0 - 36 months and 2 - 20 years in United States with Down syndrome. Includes weight, length, head circumference, weight for length (for 0 - 36 months), and BMI for age (for 2 - 20 years).

On testing, the results seem to match the published work pretty well, but there are definitely small deviations that I am attributing to the use of only 2 significant digits of precision in the reference parameters in the publication supplementary materials. So, please don’t be surprised if the percentiles are 1 - 2% off.