Iris-ued

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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.

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* Fixed an issue where data reduction would freeze when using more than one CPU;
* Removed the auto-update mechanism. Update checks will run in the background only;
* Fixed an issue where the in-progress indicator would freeze;
* Moved tests outside of source repository;
* Updated GUI stylesheet to QDarkStyle 2.6.6;

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* Added explicit support for Python 3.7;
* Usability tweaks, for example more visible mask controls;
* Added the ability to create standalone executables via PyInstaller;
* Added the ability to create Windows installers;

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* Due to new forced image orientation, objects on screens were not properly registered (e.g. diffraction center finder).