Refactor `..SeriesWithWildcards` out to `aggregateSeriesWithWildcards` and support empty `*positions`

This does two things:

Adds support to pass an empty *positions to aggregateSeriesWithWildcards. This matches what graphite web does.
- Note: I made sure other functions using GetIntArgs are OK with this change.
Move sum/multiply/average SeriesWithWildcards to aggregateSeriesWithWildcards's implementation. There was a lot of repeated code between them. Note that this is what GraphiteWeb does, see https://github.com/graphite-project/graphite-web/blob/e7d08e6af65c47c5a5dd6421c5b352d7ac3d381c/webapp/graphite/render/functions.py#L382-L403 . I think this will make it easier to be graphiteweb-compatible.
- Note: I just moved the tests from each file to aggregateSeriesWithWildcards's tests and they passed :+1:

go-graphite / carbonapi