Minor bug fixes and enhancements

• Fixes bug with fixed-length strings. If a link target is stored as a fixed-length string, it needed to be converted to unicode before output.
• Adds support for the default attribute. This can be used to identify the default entry and data groups within a NeXus file, e.g., for choosing what to plot.
• Improves error messages when plotting from a Python shell. Previous error messages could obscure the real issue, e.g., that 3D plots cannot be plotted without selecting a 2D slice.