Fix BUG when user tries to split a pane graphically.

Bug was:

Traceback (most recent call last):
  File ".../oalab/src/openalea/oalab/gui/splittablewindow.py", line 871, in _onSplitRequest
    SplittableUI._onSplitRequest(self, paneId, orientation, amount)
  File ".../oalab/src/openalea/oalab/gui/splitterui.py", line 716, in _onSplitRequest
    self.splitPane(fake, paneId, orientation, amount)
  File ".../oalab/src/openalea/oalab/gui/splitterui.py", line 477, in splitPane
    self.computeGeoms(paneId)
  File ".../oalab/src/openalea/oalab/gui/splitterui.py", line 558, in computeGeoms
    self._g.visit_i_breadth_first(visitor, baseNode)
  File ".../oalab/src/openalea/oalab/gui/splitterui.py", line 311, in visit_i_breadth_first
    ignoreFirst, ignoreSecond = visitor.visit(currNode)
  File ".../oalab/src/openalea/oalab/gui/splitterui.py", line 885, in visit
    direction = self.g.get_property(vid, "splitDirection")
  File ".../oalab/src/openalea/oalab/gui/splitterui.py", line 276, in get_property
    raise BinaryTree.PropertyException(vid, key)
openalea.oalab.gui.splitterui.PropertyException: No splitDirection for node 2

Just moving classmethod to mother class solve problem ...