Support more complex score evaluations

[chrisala]

The requirement for the MERIT organisation dashboard is to be able to:

1. Create a value calculated by the sum of one report field divided by the sum of a different report field AND
2. Create a value calculated as the sum of one report field divided by a field on the Organisation object itself.

Hence we need to be able to:

1. Evaluate expressions in Scores (e.g. val1/val2)
2. Sum distinct values (e.g. only sum the value of the Organisation once for each distinct Organisation encountered, even if the same Organisation has many reports and hence is encountered many times during the process).