Iris-ued

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* Re-licensing `iris-ued` to GPLv3.
* Changed the default colormap for processed datasets, to visually distinguish between raw and processed data viewers
* Added support for Python 3.9

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* Fixed an issue where a broken plug-in would crash Iris. Instead, broken plug-ins will not be loaded.

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* Added the `DiffractionDataset.time_series_selection` method, which allows to create time-series integrated across an arbitrary momentum-space selection mask.
This allows to create time-series from shapes that are not rectangular, at the expense of performance.
* Added a few methods to create selection masks: `DiffractionDataset.selection_rect`, `DiffractionDataset.selection_disk`, and `DiffractionDataset.selection_ring`.
* Added the ability to show/hide dataset control bar;
* Added the ability to export time-series data in CSV format;

* Fixed an issue where calculations of time-series, relative to pre-time-zero, would raise an error.
* Symmetrization dialog is no longer in "beta".

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* Official support for Linux.
* Plug-ins installed via the GUI can now be used right away. No restarts required.
* Added the `iris.plugins.load_plugin` function to load plug-ins without installing them. Useful for testing.
* Plug-ins can now have the ``display_name`` property which will be displayed in the GUI. This is optional and backwards-compatible.
* Siwick Research Group-specific plugins were removed. They can be found here: https://github.com/Siwick-Research-Group/iris-ued-plugins
* Switched to Azure Pipelines for continuous integration builds;
* Added cursor information (position and image value) for processed data view;

* Fixed an issue where very large relative differences in datasets would crash the GUI displays;
* Fixed an issue where time-series fit would not display properly in fractional change mode;

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* Added logging support for the GUI component. Logs can be reached via the help menu
* Added an update check. You can see whether an update is available via the help menu, as well as via the status bar.
* Added the ability to view time-series dynamics in absolute units AND relative change.
* Pinned dependency to scikit-ued, to prevent upgrade to scikit-ued 2.0 unless appropriate.
* Pinned dependency to npstreams, to prevent upgrade to npstreams 2.0 unless appropriate.

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* Fixed an issue where the QDarkStyle internal imports were absolute.