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.
Add a dicomfind program (still under development).
Flush dicomfind output after each directory.

This allows the user to see the results produced thus far.
Enable the "-o" option in dicomfind.
Make dicomfind return results in requested order.
Add library "dicomcli" for CLI utility functions.
Add "/" separator to query files for tag paths.
Use query to filter parser results in dicomfind.
In dicomfind, match ReferencedFileID to the filename.
Remove the Query method from Item and MetaData.

This is no longer needed, since the parser itself handles queries.
It might be brought back later, but note that its handling of nested
data sets (items) was never finished.