Minor bug fixes

• Adds backup and restore functions to the NXroot group, allowing NeXus files to be saved to disk and recovered if necessary. By default, the backup is saved in the current directory with a randomized string added to the original filename, so that it is easy to clean up unneeded backups.
• Improves handling of missing axes.
• Fixes a bug in setting NXlinks to be the signal.
• Corrects an inconsistency in the way that the axes attributes are set by either NXdata group creation or using the nxaxes property.
• Improves output of string arrays when printing NeXus trees.
• Improves Python 3 compatibility.