A search filter base class
This doesn't seem to be being used just yet. But here is some info on filters:
Filtering means imposing additional restriction on the hit list to eliminate hits that otherwise would be included in the search results.
There are two ways to filter hits:
Search Query - in this approach, provide your custom filter object to the when you call the search() method. This filter will be called exactly once to evaluate every document that resulted in non zero score.
Selective Collection - in this approach you perform the regular search and when you get back the hit list, collect only those that matches your filtering criteria. In this approach, your filter is called only for hits that returned by the search method which may be only a subset of the non zero matches (useful when evaluating your search filter is expensive).
This must be defined in a subclass