Algorithm for calculating height, weight and BMI centiles for CDC data

Example percentiles (3, 5, 10, 25 etc) are also coded into the calculate_xxx_centile functions so can be outputted for use in displaying extra information for context.

At the moment can only use age in months from 24-240. The month that is looked up in the table might be off by one as I wasn't sure how to deal with the half-integer values in the table. When the UK90 tables arrive will probably need to change all of the calculate_xxx_centile functions anyway.