Nexpy

* Fixes a bug with plots using Pyplot.

* 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.