BMI SDS and Centile

[eatyourpeas]

BMI is a calculated field using height and weight

Calculation possibly more complicated because I think it is probably valid to use the most recent weight and most recent height to calculate, even if not measured on the same day.

Otherwise this value and the most recent date should be passed to RCPCHGrowth dGC API to return an SDS and centile
In time we should do the same for the blood pressures