Fix typo in TestDICOMDirectory series information.

It was using the study index to query the series information.
Add beginnings of a Query method for data sets.

Given an object that describes a query, this will return the result
of that query.
Change vtkDICOMDirectory to return arrays rather than ranges.

This provides more flexibility in how studies can be ordered.  In particular,
sorting studies by date can be more useful than sorting by patient in some
circumstances.
Add very crude query code to vtkDICOMDirectory.

This simply allows the results to be filtered, based on whether they
match the given query.
Add a dicomfind program (still under development).