Nexusformat

* 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.

- Fixes a bug when setting a group's class, using the `nxclass` setter, causes an infinite recursion.

- Fixes some issues with defining the filenames of external links. Now, a new keyword, `abspath`, has to be set to True when initializing a link if the absolute file path is to be used. Otherwise, the relative file path with respect to the file containing the link is stored, whether the input file path is absolute or not. The default value is `abspath=False`.
- Fixes some issues related to defining floating point slices when the axes are stored as bin boundaries rather than bin centers.

- Ensures that the read/write mode of the HDF5 file is properly updated.
- Fixes the insertion of internal links into external files both as links and as copied values.
- Allows in-place saves of NXroot objects. Before, it had been necessary to copy the new NXroot object returned by the `save` function.