DICOM: fix dimension population for multi-series filesets

Fixes http://trac.openmicroscopy.org.uk/ome/ticket/12369.  I was only able to reproduce the problem in the ticket artificially - there need to be two filesets in the same parent directory (though they can be in different subdirectories), with the same acquisition date/time but different DICOM series numbers.  Temporarily modifying ```DicomReader``` to set ```originalSeries``` to ```2``` on line 1086 and opening one of the files from QA 9553 showed the stack trace without this change, and showed 2 series and no exception with this change.

/cc @kennethgillen for verification in the wild