Infrastructure for superpositions

[cscherrer]

Users can build a SuperpositionMeasure in a few ways. We need a + b and sum((a,b)) to result in the same thing. I think we should have one method to handle both cases. Whether that's a method on Tuples or a ... method mostly comes down to performance and which is more natural to code.

There's a fair amount more to do for general superpositions. In the end we need a data structure that encodes how the base measures are related, storing everything in a tree structure. That will make it easy to compute things efficiently, since it will let us share log-density computations. It will also make it easier to fold more measures into an existing superposition.

We'll also need to build the logmass API we've talked about before.

Originally posted by @cscherrer in https://github.com/cscherrer/MeasureBase.jl/pull/46#discussion_r863853741