Add external links to groups

• It is now possible to create an external link to a group as well as a field. External links may be initiated at any time but they are only available within a NeXus tree when the data are saved to a file.
• Fixes a bug affecting the use of scalar NXfields in Numpy functions expecting arrays.
• Adds _backup_ after the original file name and before the random text to make it easier to identify the original name.