CellVoyagerReader: warn if we have to open an unknown file version

I bet two smarties that the device we got (at the Pasteur imaging
facility) is going to be severely updated and that most of the
problematic XML tags will get fixed. Since I cannot foresee the future
that well, I put a warning to future users.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>