Pdsview

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* Fixes bug in ROI creation

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* Use QtPy and do not depend on PySide
* Create a histogram to apply cut levels

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* Updated to work with ginga 2.5.
* Bug fixes.

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* Clicking on a point in an image now shows the image coordinates and pixel
value at that location.
* Right clicking and dragging on the image will now create a region of interest.
Statistics about that region of interest are shown.
* Composite RGB images can now be displayed by using the new Channels dialog and
checking the RGB toggle.

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* Reworked to use an ImageSet and ImageStamp model to improve program flow and
permit launching from other programs.

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* Cleaned up with parts rewritten.
* Takes files as command line arguments.
* Handles mutiple images.