Add test for ats

Ats is similar to at, but instead of just doing at the specific path,
this does it on each of the direct children of the particular path and
returns a list (array) of scoped models of each of those children.
Implement ats

Convenient function for getting all at's from a list
Update test name
Add tests for adding ats functionality to filter
Move filter.context to a more appropriate place in the code
Add filter.ats -functionality

Now passing all tests