Non-simple expressions in matching score are re-evaluated for each individual

[gdementen]

Non-simple expressions (EvaluableExpression children) are re-evaluated on all of set2 for each individual in set1. For example, links in the score expression (as in tests / functional / simulation.yml) are recomputed over and over again. Needless to say, this slows things down tremendously. We should detect them and pre-evaluate them. Or... just wait for caching and "common subexpression elimination" (#98) which should fix this issue nicely.