Add functions to calculate z-scores and centiles for various newborn anthropometric indices based on the INTERGROWTH-21st standards

[ernestguevarra]

From @ernestguevarra on May 25, 2018 8:56

Using the INTERGROWTH-21st reference standards, develop function/s to calculate newborn:

[ ] birthweight centiles and z-scores
[ ] length centiles and z-scores
[ ] head circumference centiles and z-scores
[ ] weight-length ratio centiles and z-scores

Copied from original issue: nutriverse/zscorer#11