Nexusformat

* 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.

* An attribute that is written as a size 1 array is now converted to a scalar. This fixes the reading of size 1 string arrays.

* If a NeXus file is unlocked, it is now possible to modify the values of any linked items, not just the parent item. External links are still read-only.

* Fixes a bug when plotting data with bin boundaries sliced with an unlimited maximum value.
* Fixes a bug when invoking the plot method on an unsaved root group.
* Fixes a bug with updating files with new links.
* Adds a new module for opening files stored on remote servers using `h5pyd`.

- Fixes bug with fixed-length strings. If a link target is stored as a fixed-length string, it needed to be converted to unicode before output.
- Adds support for the default attribute. This can be used to identify the default entry and data groups within a NeXus file, _e.g._, for choosing what to plot.
- Improves error messages when plotting from a Python shell. Previous error messages could obscure the real issue, _e.g._, that 3D plots cannot be plotted without selecting a 2D slice.