Class PatternCountScreen

BaseScreen.BaseScreen --+
                        |
                       PatternCountScreen

Expects as input (besides the oemolistream to screen and 
the oemolostream to output to) a SMARTS file describing the
patterns to screen by.  One SMARTS pattern string is expected
for each line of the file.  Furthermore, each SMARTS line should
end with an additional token (whitespace separated) representing
an integer "count."  If not present (or the last token is not 
parsable as an integer), then a default value of ">=1" is assumed.

A molecule passes this screen iff
for each SMARTS pattern listed, 
    the molecule contains exactly "count" instances of that pattern
    
For example, a SMARTS file like the following:

    O=C[OH] CarboxylicAcid  2
    [NH2]   0
    [Sn]    Tin
    
Will screen for all molecules that contain exactly 2 carboxylic 
acids, 0 primary amines and at least 1 tin atom.  Note that the 
pattern label is not actually important (none specified for 
primary amine) and that if the count is ommitted (as in for Tin 
above), this is treated as a special case of "at least" rather
than "exactly" 1.