DataList had several methods that should act on a copy and return
that copy, but was instead mutating the existing list.
We cant change this behaviour in the 3.0 line for backwards compt.
reasons, but this makes the desired behavior the default, and
makes disabling the mutation in 3.1 easier
It also introduces two new methods to deal with the common pattern
of wanting to modify the underlying dataQuery, which we want to be
able to reliably do in a way that always acts immutably. The main
method of these two is alterDataQuery
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