Add a SetQuery method to vtkDICOMParser.

This allows the parser to scan a file for certain attributes, and
to provide a yes/no answer as to whether a file matches a query.
Flush dicomfind output after each directory.

This allows the user to see the results produced thus far.
Use parser-based query in vtkDICOMDirectory.

In vtkDICOMDirectory, the parser is used as a scanner that only reads
data elements that are of interest.  This improves the efficiency.