Nexusformat

Version 0.4.15 contained a serious bug affecting the handling of NeXus attributes, which has triggered a number of changes to how the package handles byte string attributes.

* Adds the `nxvalue` property to both NeXus attributes and fields, *i.e.*, NXattr and NXfield objects. Whereas the `nxdata` property contains the value in the original form that is stored in the NeXus file, the `nxvalue` property converts all byte strings to (unicode) strings, string arrays to lists of strings, and size-1 arrays to scalars. This allows Python scripts to utilize the stored values consistently, without being affected by low-level details of how they are stored in the HDF5 files.
* Fixes a bug reading NeXus attributes containing byte string arrays.
* Fixes a bug where empty 'NX_class' attributes are added when reading groups with no NeXus class specified.
* Fixes a bug when the target attribute of a linked object is stored as a byte string.

* Updates the list of NeXus base classes.
* Improves the `nxaxes` property for NXfields that are not default signals. It now returns the same as the group's default axes if the dimensions are compatible. Multiple signals within a group will now be over-plotted on the same axes.
* Allows NXlinks to be used as signals and axes when creating an NXdata group.
* Adds the `uncertainties` attribute to signals if an error field is specified when creating an NXdata group.
* Automatically converts NXentry groups to a class of NXsubentry when they are assigned to other groups. NXentry groups should only be at the top level of a NeXus file.
* Adds `nxversion`, set to the API version number, to the attributes imported by default.
* Fixes a bug with the file paths of external links embedded in another external link (Fix for 51)
* Fixes a bug when saving external links with absolute file paths.
* Makes the `maxshape` attribute compatible with scalar fields.
* Improves consistency of storing fixed-length string arrays as attributes.

* Adds ability to use NXdata slicing to set and remove masks. This also removes the mask attribute and field from the signal when none of the data is masked and fixes a bug in setting masks.
* Modifies the `short_tree` function to display the attributes of the first item. This makes the NeXpy tree tooltips more useful, without increasing their length substantially.
* Improves handling of externally linked data pointing to non-existent files.
* Fixes a bug setting NXfield properties.
* Fixes a bug using the NXfield sum command with multiple axes.

* Traps `h5py` exceptions when an HDF5 file contains invalid attribute values. This ensures that the HDF5 file will still load.
* Improves writing of generic (*i.e.*, non-NeXus) HDF5 files by removing unneeded class attributes.
* Adds a Jupyter Notebook tutorial to the distribution.

* Fixes a bug when transposing axes in data projections, caused by the restrictions on changing the shape of an existing group item.
* Fixes a bug when renaming NXroot objects
* Fixes a bug in reading axes defined by byte strings
* Ensures that plotting axes that are also NXlinks do not use the parents' name, since these may be generic.
* Makes deep copies of linked items use the children's name, for similar reasons to the previous issue.
* New `sum` and `average` functions are added to NXfields.

* Fixes a bug with renaming fields or groups when the data have not been saved to a file.
* Improves error message when referencing missing attributes in an object without a parent group.