DICOM: fix file grouping so that path names are normalized

On Windows in particular, the list of pattern files often has extra path separator escaping, which prevents the
pattern files from being matched with the files from the directory listing.
Merge pull request #1049 from melissalinkert/rebased/develop/dicom-file-grouping

DICOM: fix file grouping so that path names are normalized (rebased onto develop)