…y files changed:

- Viewer class format property changed to 'PNG', format property removed from child classes.
- Viewer class options property changed to {"compress_level": 1}, format property removed from child classes.
- IPythonViewer format option set to None, IPythonViewer options set to {}.
- Corresponding test Tests/test_imageshow.py::test_viewer changed to meet the changes mentioned above.

- In the current implementation of UnixViewers threads were used in order to control opening/closing of images by external Linux/Unix viewers.
- The Python program remains opened till the user closes the image in the viewer.
- I used viewer thread and monitoring thread, the monitoring thread is responsible for removal of the temporary images after user closed the image in the viewer.
- Corresponding documentation is updated.
- Tests are updated: Tests/test_imageshow.py, Tests/helper.py

Test skip reason have been corrected.

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