Abstract base class providing a mechanism to restrict searches to a subset
of an index and also maintains and returns position information.
This is useful if you want to compare the positions from a SpanQuery with the positions of items in
a filter. For instance, if you had a SpanFilter that marked all the occurrences of the word "foo" in documents,
and then you entered a new SpanQuery containing bar, you could not only filter by the word foo, but you could
then compare position information for post processing.