Changes:
* guiqwt now requires guidata >=v3.0.
* tests/syncplot: added image synchronization test
Bug fixes:
* [Issue 107](https://github.com/PlotPyStack/guiqwt/issues/107) - App with multiple ImageWidgets seg faults about 40+% of the time on startup
* [Issue 104](https://github.com/PlotPyStack/guiqwt/issues/104) - TrImageItem will emit a signal when the image is moved but not when resized
* [Pull Request 103](https://github.com/PlotPyStack/guiqwt/pull/103) - use np.isnan() to handle if image data contains nan values, by [RussBerg](https://github.com/RussBerg)
* [Issue 102](https://github.com/PlotPyStack/guiqwt/issues/102) - image.py: get_histogram() will fail if image initialized with Nan values
* [Pull Request 101](https://github.com/PlotPyStack/guiqwt/pull/101) - corrected arguments names for specifying image center, by [RussBerg](https://github.com/RussBerg)
* [Issue 100](https://github.com/PlotPyStack/guiqwt/issues/100) - Images: changing X-axis direction does not work with autoscale
* [Issue 99](https://github.com/PlotPyStack/guiqwt/issues/99) - trimage builder specifies wrong attributes for altering image center
* [Issue 98](https://github.com/PlotPyStack/guiqwt/issues/98) - numpy >= 1.20 incompatibilities
* [Issue 90](https://github.com/PlotPyStack/guiqwt/issues/90) - Image visualization: should first pixel be centered at (0,0)
* Fixed coords inversion in EllipseShape (circle)
* tests/syncplot: fixed plot rescaling at initialization