Scikit-ued

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This release brings major infrastructure improvements, which in turn have allowed to squash some bugs.

* Migration of continuous integration testing to GitHub Actions.
* Migration of test infrastructure to pytest.
* Tests are now included in source distributions.
* Code snippets in documentation are now tested for correctness.

User-facing changes:

* Fixed an issue where the detected shift in `skued.align` and `skued.ialign` might be partial (i.e. only shift in one direction).
* Fixed an issue with packaging data on Linux.
* The `fast` argument to `skued.align` and `skued.ialign` has been removed. It was previously-marked as deprecated.
* Added pre-emptive support for Python 3.10+ by removing deprecations.
* Increased the precision of the pseudo-voigt approximation in `skued.pseudo_voigt`.
* Fixed many issues regarding documentation being out-of-date.

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* `scikit-ued` is being re-licensed from the MIT license to the GPLv3 license.
* The `fast` argument to `skued.align` and `skued.ialign` has been deprecated. Its value has no effect anymore.
* Official support for Python 3.9.
* Removed explicit requirement for the `tifffile` package.

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* Added `skued.kinematicsim`, a simple function to compute electron diffraction patterns from
crystals structures in the kinematic approximation (i.e. thin samples).
* Added the `skued.RingArcSelection` area.
* Various documentation improvements and fixes.

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* Added support for `crystals.ElectronicStructure`. This requires `crystals` version 1.1.0 and up.
* Added the function `with_irf`, which allows to modify fitting functions to include the effects of instrument response.
* Various documentation fixes.

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* Added the `Selection.mpatch` method to draw patches on Matplotlib plots.
* Added the `spectrum_cmap` Matplotlib colormap, available under the name `"spectrum"`.
* Fixed an issue where diffracted intensities were not correctly scaled in `potential_map`.

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* Added the :class:`Selection` class and :class:`RectSelection`, :class:`DiskSelection`, :class:`RingSelection`, and
:class:`ArbitrarySelection` to assemble time-series. This is a generalization of iris-ued's time-series rects.
* Added real-time pixel value and cursor position to ``skued.diffshow``.
* Added `indices_to_text`, a plotting utility function to render Miller indices to Mathjax/LaTeX-style text (Matplotlib-compatible).