Fix a bug in the vtkDICOMItem::Clear() method.

If the reference count was greater than one, then the Clear() method
had no effect.  Now it does.
Add a vtkDICOMDirectory class for DICOMDIR handling.

This class will look for a DICOMDIR first, but if no DICOMDIR is found
then it will search the directory file DICOM files.
Change the sorting order for DICOM directory listings.

Sort patients by name, rather than ID.
Sort studies by date, rather than UID.
Sort series by number, rather than UID.
Make the vtkDICOMDirectory sorting more efficient.
Add methods to get records from vtkDICOMDirectory.
Add a test for vtkDICOMDirectory.
Avoid reporting dirextories with double "/" at end.

If SplitPath splits a directory that ends with "/", then it leaves
an empty string at the back of the split.  Remove the empty back if
this occurs.