Fixed a major FireVoxel-wide DICOM error.  When encountering the DICOM tag of VR_TM type, the .Fractions (=microseconds) field was incorrectly converted to string when the number of digits in .Fractions was <6.  Fix was to pad the Fractions with 0s to contain 6 digits corresponding to microseconds.  This might have affected all the 4D volumes.