Store attribute values as array instead of set

[tokebe]

First described here

Because we store attribute values as a set, any value that is supposed to be an additive number (for instance evidence_count) can be added incorrectly if there are duplicate values (as they'd be squashed by the set).

Instead of this, we can change attribute value storage to an Array, and then handle set cases for array-type TRAPI attributes in our TRAPI attribute generation.

[tokebe]

Medium-low priority.

[rjawesome]

see pr