Makes NXPlotView shape and ndim properties

This removes dimensions of size 1 from the plotting shape and number of
dimensions.
Fixed PlotDialog functionality

‘Plot line’ would still plot points if PlotDialog was triggered.
Improved contextual menus in NXTreeView

This has required adding plot_rank and plot_shape properties to
NXfield, in which the rank and shape have size-1 dimensions removed.
Fixed plotting of individual NXfields

If there were size-1 axes, they were not properly handled in the
PlotDialog. This also revealed a bug in NXfield._setdata (fixed in the
last commit), that meant that, when _uncopied_data was set, it wasn’t
copied to _value if the new field was not stored in a file.