WB-860 NIFTI Volume Timeseries Issues: VolumeFile was always reporting...

WB-860 NIFTI Volume Timeseries Issues: VolumeFile was always reporting NIFTI_UNITS_UNKNOWN as it failed to override CaretMappableDataFile::getMapIntervalUnits().  VolumeFile now overrides getMapIntervalUnits() and it returns NIFTI_UNITS_SEC if the time dimension is greater than one and NiftiHeader::getTimeStep() is greater than zero.