Add a 'new_collection' method
Add a 'new_collection' method

Generate a new Collection from the given Entities. I toyed with making
this public, as I suspect it'll be more generally useful, but let’s
leave that until we actually have a use for it. Deferring that also
gives us time to find a better name!

everypolitician/everypolitician-popolo,everypolitician/everypolitician-popolo