fix to handle year 0000 in ParaView VTK Extractor

[irenavankova]

In ParaView VTK Extractor: fix to handle year 0000 as a special case, datetime() doesn't understand year 0000

[xylar]

@irenavankova, looking through the code for the paraview extractor, if you supply --xtime=none instead of --ignore_time, that will extract all the time levels but won't look at the xtime variable. That would be my suggestion for how to handle this case.