utility: pathHelpers: getPathVariants() interprets url paths as 'absolute'
OpInputDataReader: FilePath input can be a dvid URL.
submodules: Updated dvidclient
submodules: updated dvidclient
tests: Fixed testOpDvidVolume
pathHelpers: quick-test checks that everything works for urls, too.
OpInputDataReader: Minor import cleanup.
Added OpExportDvidVolume.  Used it to add dvid support to OpExportSlot.
opExportDvidVolume: Added transpose_axes arg.
opExportSlot: When exporting to DVID, assume the user provided C-order axes.
OperatorMetaClass: Comment.
OpExportSlot.FormatSelectionIsValid now supports dvid.
OpInputDataReader: Raise DatasetReadError if dvid volume can't be found.
OpDvidVolume: init client in _after_init()
Updated dvidclient.