Dvg-pyqtgraph-threadsafe

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* Workaround: ``PyQt5 >= 5.12.3`` causes a bug in ``PyQtGraph`` where a
curve won't render if it contains NaNs (but only in the case when OpenGL is
disabled). The curve will now be displayed correctly, i.e., fragmented
whenever a NaN is encountered. When OpenGL is enabled, linear interpolation
will occur at the gaps as per ``pyqtgraph.plotCurveItem.paintGL()``.

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* Method names are now conform the ``PyQtGraph`` naming style. I.e.
``setData()`` vs. ``set_data()``, etc.
* The default values of ``PyQtGraph`` are no longer being overwritten.
* Added class ``LegendSelect``

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* First release on PyPI