Nexusformat

* Fixes an issue with missing links in external files.

* Fixes an issue preventing NeXus trees from being pickled.
* Fixes an issue when using special formatting instructions to print NXfields.
* Officially advertises support for Python 3.10.

* Adds a new NXvirtualfield class to implement HDF5 virtual datasets.
* Checks for virtual fields when loading HDF5 files.
* Adds a new `nxconsolidate` function and command-line script to consolidate NXdata groups from multiple files into one NXdata group containing a virtual dataset.
* Adds a new `nxdir` script to print a NeXus file tree from the command line.
* Adds support for qualitative color maps in the `plot` module, with alignment of integer values to the discrete color bands.
* Converts package to use the latest recommendations for package distribution, such as creating a `pyproject.toml` file to configure the build backend, moving metadata and requirements from `setup.py` to `setup.cfg`, and using `setuptools_scm` for version management.
* Improves PEP-8 compliance of code.

* Allows the `nxsignal` and `nxaxes` values to be set by the field names as well as by NXfield objects.
* Allows the assignment of NXlinks to `nxsignal`.
* Improves reporting of invalid values when setting NXdata properties.
* Corrects NXdata weights when summing data.
* Enables plotting of size-1 arrays.
* Fixes a bug in identifying projection limits of zero range.

* Adds support for the NeXus definition of data weights defined by the property `nxweights`. A NXdata function, `weighted_data` returns the signal divided by the weights.
* If the `plot` function is called with the keyword argument `weights=True`, the data are plotted with the weights applied to the signal.
* If the `plot` function is called with the keyword argument `aspect=equal`, the axes are scaled by the value of the axis attribute `scaling_factor` if it is present. For example, this corrects the aspect ratio to be equal in absolute units when the axis values are in lattice units or reciprocal lattice units.
* Adds the NXdata `select` function to return points from 1D NXdata groups that are divisible by a specified number, optionally combined with an offset value. The default returns the values at all integer axis values (i.e, `divisor=1`, `offset=0`).
* Adds extra keyword options to the `plot` function (e.g., `aspect`, `cmap`, `interpolation`, `bad`) to enable plots to reproduce NeXpy options more faithfully.
* Allows the 1D smoothing to be set as a multiple of the original axis steps to ensure that the smoothed interpolations include the original points.
* Fixes a bug where a NXdata slab, whose signal is stored in an external file, is returned with the signal name changed to the field name in the external file.

* Improves checks of file permissions when opening NeXus files.
* Fixes an issue preventing plots containing NXfield links that are not saved to a file.
* Ensures that the `nxerrors` property only returns a result if it matches the shape of the `nxsignal` property.
* Checks for zero range in NXdata projection limits.
* Returns the length in bytes of a scalar string, rather than the array size, with the `len` function.
* Adds an `nbytes` function to NXfields.
* Removes deprecated NumPy dtypes.
* Removes change to the 'bad pixel' color when plotting. Changes to Matplotlib color maps are deprecated in Matplotlib v3.4.