ENH: Adding the capability to process derived items

DerivedItems is a list of file names attributes on the
loadable with the DICOM instances produced by the plugin,
which will be added to the database after loading
the loadable.

From: Andrey Fedorov <fedorov@bwh.harvard.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23546 3bd1e089-480b-0410-8dfb-8563597acbee
BUG: Create new indexer for each file

Suggested by Steve Pieper as a way to work around potential issues in CTK

From: Andrey Fedorov <fedorov@bwh.harvard.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23547 3bd1e089-480b-0410-8dfb-8563597acbee