New rule to get aggregate cover value for a postcode area

This would take the form of two new conditions in rules builder:

GetAggregateCoverForPostcode() and GetAggregateCoverForClientPostcode()

The functions would take as arguments: 
- a coverage question name from the current question set (this would not be required for GetAggregateCoverForClientPostcode())
- an operator from the usual list (greater than, less than, etc.) 
- a numeric value entered by the user

This could be used to generate referrals if cover limits were exceeded for given areas.