Sketch aggregate.multi_epidist() method

[Bisaloo]

This is the result from our discussion at the Epiverse - PERG development day. Key contributions were made by Anne Cori, Sangeeta Bhatia, Patrick Doohan, Christian Morgenstern, Finlay Campbell and they should probably be all listed either as function authors, package contributors, or both.

I went with an aggregate() method for this but feel free to link it to another generic if it makes more sense for you.

[joshwlambert]

Thanks @Bisaloo and all other contributors. I like the idea, and the documentation and skeleton code make it clear the intention of the function. I have opened #271 to host the discussion around design decisions for this function.