Nexpy

* Fixes an issue with changing axes in the Limits Panel.
* Fixes a bug in the 'chopper' example plugin.

* Adds support for pre-defined 'models' in the lmfit package in the Fit Panel. From now on, users are encouraged to define their own fitting models as `lmfit` Model classes, although legacy NeXpy function modules are still supported.
* Adds miscellaneous improvements to the Fit Panel, including selection of fitting methods and tooltips to display model definitions.
* Adds a 'Fit' button to the Y-tab of all one-dimensional plots to initiate fits of plotted data.
* Enhances the Export Dialog to allow any NXdata group to be exported as a NeXus file or, for one-dimensional data, an ASCII file.
* Adds a button to the Options Tab to export the plotted data.
* Allows the order of legend entries to be changed in the Customize Panel.
* Adds additional color maps.
* Improves plot updates in the Limit Panel.
* Corrects the radius setting when plotting circles.
* Fixes a bug when plotting data with step sizes much smaller than their absolute values.
* Fixes a bug causing intermittent crashes when using 'Ctrl+W' to close dialog windows.
* Fixes a bug causing intermittent PyQt crashes when closing NeXpy.
* Fixes compatibility with the upcoming Matplotlib v3.3 release.

* Improves behavior of axis limit boxes and sliders. The slider limits and sensitivity are only reset when a user explicitly edits the limit boxes.
* Uses the `qtpy` module to import PyQt libraries. This fixes a bug when running in 'headless' mode over SSH, *i.e.*, when X-window forwarding is not enabled.
* Updates plots when a user clicks `Copy` in the Limit Panel. The `Apply` button has been removed.
* Filters out files with invalid external links when choosing files in the Scan Panel.
* Improves initialization of contextual menus in the tree pane.
* Removes minor ticks from color bars with log scales. Minor ticks for both the axes and color bars can be turned on and off in the Customize Panel for images. For 1D data, type `plotview.minorticks_on()` and `plotview.minorticks_off()`, respectively, in the IPython shell.
* Requires `nexusformat` v0.5.2 because of internal changes to the API.
* Fixes a bug with the data tables when using `View Data`.
* Fixes an incompatibility with Matplotlib v3.2.
* Adds requirement for v5.2 of `ipykernel`, which fixes a bug with in-process `qtconsole` kernels.

* Minor bug fix to correct issue with saving data from the Scan Panel before plotting.

* Allows the Projection Panel to be used with a Scan Panel plot.
* Allows the Customize and Limit Panels to be opened from Projection and Scan Panel plots.
* Adds line numbers to the Script Editor for all operating systems and improves scrolling behavior on MacOS.
* Automatically converts tabs to four spaces when saving scripts.
* Improves error messages when installing a new plugin.
* Fixes start-up bug when old backup files cannot be deleted.
* Fixes a bug in the Limit Panel that resets old values.

* Adds a new Scan Panel for combining data in multiple NeXus files into a single multidimensional array. A scalar field that changes in each file provides an additional plot axis allowing different files to be compared as a function of a parametric variable, such as temperature. The new array can be plotted and/or saved for subsequent analysis.
* In a related feature, enables parametric scans of a scalar field stored in multiple NeXus files using "Plot Data."
* Extends the Limits Panel to allow the limits of multiple plots to be synchronized. Changes to one plot will be automatically propagated to the other synchronized plots.
* Extends the Customize Panel to add the ability to scale and/or add an offset to 1D plots.
* Harmonizes keyboard shortcuts for the four panels (Customize, Limits, Projection, and Scan).
* Adds a "Cut Data" option to copy and delete a NeXus field or group. The copied objects can be pasted into another NeXus tree.
* Provides a fix for MacOS PyQt5 bugs. In PyQt5 > v5.11, a number of widgets are not repainted until clicking outside the window. This does not affect PyQt5 on Linux.
* Adds support for Python 3.8.