Nexusformat

* Generalizes the NXdata `moment` function to compute moments of arbitrary order of one-dimensional data.
* Adds a cell to install `nexus format` if necessary in the Jupyter Notebook tutorial.
* Improves casting of array-like values when creating NXfields.
* Allows `maxshape` to be defined when creating an NXfield.
* Fixes a bug with plotting NXdata groups with size-1 axes.

* Reads HDF5 soft links as NXlink objects. NeXus only uses hard links, but this allows non-conforming files to be loaded.
* Improves selection of default axes when plotting NXfields.
* Improves the printing of NeXus trees that contain long string attributes and unresolved links.
* Improves handling of null values in NXfields.
* Allows full paths to NeXus objects to be used when checking for keys in group dictionaries.
* Fixes the behavior of NXfield iterator functions when the value is not iterable.

* Fixes a bug with creating NXdata groups with one-dimensional Numpy arrays.

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.