Change Growth Monitoring Calculation

http://manage.dimagi.com/default.asp?139222 (child of http://manage.dimagi.com/default.asp?139090)
Includes https://github.com/dimagi/commcare-hq/pull/4435, moved here so I could add minor modifications.
@czue
where is this used? does a reference need to get updated somewhere else? cc @msienkiewicz