Add rudimentary time handling to dicomtonifti.

It is "rudimentary" because the time support in the DICOM reader
is still rudimentary.  The TimeSpacing is always one, and the reader
does not differentiate between time frames and other frame types.
Flush small floats to zero to avoid "-0" in header.
Clean up the time handling in vtkDICOMReader.
Add SetDescrip back to vtkNIFTIHeader.