Nexusformat

- Fixes a bug when setting a group's class, using the `nxclass` setter, causes an infinite recursion.

- Fixes some issues with defining the filenames of external links. Now, a new keyword, `abspath`, has to be set to True when initializing a link if the absolute file path is to be used. Otherwise, the relative file path with respect to the file containing the link is stored, whether the input file path is absolute or not. The default value is `abspath=False`.
- Fixes some issues related to defining floating point slices when the axes are stored as bin boundaries rather than bin centers.

- Ensures that the read/write mode of the HDF5 file is properly updated.
- Fixes the insertion of internal links into external files both as links and as copied values.
- Allows in-place saves of NXroot objects. Before, it had been necessary to copy the new NXroot object returned by the `save` function.

- This fixes a bug at the startup of NeXpy v0.8.1 when v0.4.0 is installed. A new version of NeXpy, which does not have this bug, is imminent but not yet uploaded.

- 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.

- 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.