Location: only delegate to File.isHidden() on Windows

This bypasses substantial performance problems when calling isHidden()
on Mac OS X/Linux over NFS.  Fixes #6586, see #12027.
FormatReader: speed up getUsedFiles()

We can assume that each call to getSeriesUsedFiles(...) will contain an
array with no duplicates.  If there is only one series in the fileset,
then, we can speed up large filesets by removing the extra check for
duplicate files.

See #12027.
DICOM: ensure that getSeriesUsedFiles returns an array of unique paths
Merge pull request #1028 from melissalinkert/rebased/develop/pe-performance

Large fileset performance improvements (rebased onto develop)