Nexpy

* 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.

* Improves the handling of external links, especially if an externally linked group contains internal links.
* Improves error messages when overwriting existing fields, which is allowed if the new value has the same shape. The `replace` function must be used when the shapes do not match.
* Allows all the ways of identifying field uncertainties supported by the NeXus standard.
* Uses "." to denote empty axes in conformance with the NeXus standard.
* Fixes the parsing of `axes` attributes that generated a FutureWarning.
* Limits the number of recursions used in the `short_tree` property to two.
* Ensures that natural sorting is used in printed trees, *i.e.*, `scan_10` comes after `scan_2`.
* Fixes a bug in the `NXstack` script, which could cause missing frames at the end of the stack.

* Catch errors reading in NXfield values that prevent the loading of a NeXus file from completing.
* Fixes a bug with plotting data with linked fields.
* Fixes a bug causing infinite recursions when accessing link attributes.
* Adds an NXfield replace function to force a change in the size and/or shape of data already saved in a NeXus file.