Nexusformat

- 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.

- Improves the handling of fixed-length strings that contain non-ASCII characters. These are now stored as byte strings within the NXfield.
- In Python 2, the NXfield returns an encoded byte string for printing, rather than a Unicode string. This prevents some Unicode encoding errors.

- New NXfield properties have been defined for HDF5 parameters, including compression, chunk sizes, fill values, and maximum shapes. These can be set at any time before the data is saved to an HDF5 file or before slabs are stored in core memory files (for large datasets that are filled as slabs after creation).
- Fixes a bug in Python 3 when storing lists of strings. These were converted to Numpy arrays of kind 'U', which h5py was unable to convert. Now, they are automatically converted to Numpy arrays of variable-length strings, using the special h5py object datatype. If fixed-length strings are required, a dtype of 'S' (or 'Sn' where `n` is the required length) should be used when creating the NXfield.
- Provides better parsing of input parameters when creating an NXfield, _e.g._, the `maxshape` attribute is checked to ensure it is compatible with the NXfield shape.

- There is no new functionality in this release. The pyro directory, which implemented a prototype for accessing NeXus files remotely, has now been moved into the nexpy/nxremote repository.