Nexusformat

* 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.

This release is the first to require Python 3.

* Speeds up the loading of NeXus files by limiting recursion of group reads to two levels in the hierarchy by default. Lower levels are automatically loaded when they are referenced. Fully recursive loading can be restored by calling the `nxsetrecursive` function.
* Adds support for writing HDF5 soft links without the `target` attribute by adding the a boolean keyword argument, `soft`, when creating an NXlink. It is set to `False` by default.
* Adds support for the `interpretation` attribute. If the NXdata signal field has the attribute set to 'rgb' or 'rgba', it is plotted as an image by default.
* Changes the default text encoding to UTF-8, rather than using the system default. This is required by the NeXus standard.
* Fixes a bug in the `rename` function preventing affected `signal` and `axes` attributes from being updated.
* Fixes an issue with setting a NXfield dtype of kind 'S' without specifying its length caused by changes in h5py v3.

* Makes smart indexing of NXfields consistent with NumPy arrays.
* Improves handling of scalar values in NXfields derived from masked arrays.
* Improves formatting of floating point values.
* Improves floating point precision when computing axis values.
* Catches import errors when the readline package is not installed.

* Improves synchronization of fields or groups within a linked group. It is no longer possible to modify such items directly from the linked group.
* Improves detection of data for default plots within NXsubentry groups.
* Adds new NXgroup functions, `get_default` and `set_default` for setting and identifying default plots.
* Adds new `any` and `all` functions to NXfields, to mirror the respective NumPy functions.
* Adds new `ndim` and `shape` functions to NXdata to return the respective properties of the signal.
* Fixes a bug that allowed attributes to be updated when a NeXus file is locked.
* Fixes a bug when testing the existence of an externally linked path within a corrupted NeXus file.