[VTK] Avoid integer underflow in SubsamplingVTKWriter

If the SubsamplingVTKWriter is passed a negative subsampling level like
-1, that value creates an underflow.

Fixed by moving to a signed int and throwing an exception for negative values.
[VTK] Print out an error message if the output file cannot be created

If the VTKWriter fails to create the output file (most of the time
because the containing directory does not exist), the program fails with
a very unhelpful error message.

This patch improves the situation by at least logging to stderr that
something went wrong during the file creation before re-throwing the
exception.
[VTK] Add SubsamplingVTKSequenceWriter

The VTK support was missing a sequence writer that also does subsampling
for higher-order methods.

This patch adds the missing implementation and also consolidates the
common code for the two SequenceWriter variants in a common base class.