Add additional optional ERXKey Types to be able to easily tell if a Key represents a Operator (@sum, @flatten, etc.) or a non-model "attribute" or "relationship".
Add valueForKey(ERXKey<T>) to NSArray that returns an NSArray<T>, with options for automatic flattening, distincting, and automatic null-value removal. Greatly simplifies making type-safe calls on to-many relationships and any other NSArray.
Merge branch 'ERXKeyAdditions' of https://github.com/nekesto/wonder