Add IncrementalIndex test

[bsyk]

Test that should repro the IncrementalIndex + ComplexColumn + longSum aggregator issue

[kgyrtkirk]

I don't really see why this is beneficial at all - why can't the user invoke a method which will translate the histogram to a literal and the aggregate those values with sum?

I think an aggregate which is repesented by a sketch or something could normally not be represented by a single literal number; if we want to introduce something which translates the sketch to some other type automatically - I think that should be registered properly and not hidden in the aggregator as some bonus functionality.

am I alone with this mindset?

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