Class SimilarityScreener

object --+
         |
        SimilarityScreener

Known Subclasses:

ThresholdScreener, TopNScreener

base class

important attributes:
   probe: the probe fingerprint against which we screen.

   metric: a function that takes two arguments and returns a similarity
           measure between them

   dataSource: the source pool from which to draw, needs to support 
           a next() method
           
   fingerprinter: a function that takes a molecule and returns a
          fingerprint of the appropriate format


 **Notes**
    subclasses must support either an iterator interface
    or __len__ and __getitem__

x.__init__(...) initializes x; see x.__class__.__doc__ for signature

used to reset screeners that behave as iterators