Do not allow 2D datasets to be used for labels

Traceback (most recent call last):
  File "veusz/windows/plotwindow.pyc", line 920, in checkPlotUpdate
  File "veusz/document/doc.pyc", line 432, in paintTo
  File "veusz/widgets/root.pyc", line 123, in draw
  File "veusz/widgets/page.pyc", line 366, in draw
  File "veusz/widgets/widget.pyc", line 309, in draw
  File "veusz/widgets/graph.pyc", line 284, in draw
  File "veusz/widgets/textlabel.pyc", line 130, in draw
  File "veusz/setting/setting.pyc", line 1354, in getData
  File "veusz/document/doc.pyc", line 1049, in formatValsWithDatatypeToText
  File "veusz/utils/formatting.pyc", line 206, in formatNumber
  File "veusz/utils/formatting.pyc", line 134, in formatGeneral
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()