Currently, the zoomLevel is reset for every plotDive() call,
because the zooming is done via the QGraphicsScene. However,
this does not work well (e.g. axes are likewise zoomed) and
in the future a change of the zoom level will cause a replot.
Thus, remove the zoom-reset in plotDive() and do it explicitly
when switching dives.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>