Create UK90algorithm.py

For use with data file UK90_height_weight_bmi.csv
Input when used in command line should be ordered: HEIGHT, WEIGHT, AGE, SEX.
Note that when a positive age is used (ie not premature) AGE is in months.
When a negative age is used (ie premature baby) AGE is in weeks.
This code displays some runtime warnings when the age is below negative 7 weeks (premature babies) but I think this shouldn't be a problem since when this does occur the height and bmi data should be ignored since this is missing from UK90 tables