[new feature] OverlayImage processor

[Psylocibus]

processor that adds an image over the original image.

• Processor added in overlay.py
• test added in test_processor.py

[Psylocibus]

I need that feature to add watermark image easily.
I may need a review for the "return" of the process method : for now, I copied the style of return of ColorOverlay : the returned image is the image in input. But I just saw it is not the case everywhere (sometimes a new_image is reurned).

[vstoykov]

Thanks @Psylocibus for the nice contribution!