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

[ernestguevarra]

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

[ernestguevarra]

This issue was moved to nutriverse/intergrowth#15