Allow indicators to be configured

Indicators results can vary a lot depending depending on how they are coded. If we want to facilitate reusable indicators without forcing people to extend classes we should allow people to configure them up to some extent. e.g. how many accesses per day is considered a lot of accesses?

This issue is complex and would require significant changes, but on the other hand to formalise indicators tuning would help us automate testing of different models by tuning not only the list of indicators but the indicators itself.

I've realised of how important this issue could be by working on a model and reusing core indicators.