remove magic, updated viewer, fix colormap

[guiwitz]

Few fixes in napari notebook:

• The magic command for qt is not needed anymore.
• No colormap was actually used as said in the text
• The viewer screenshots now show the current UI
• typos

and in nd image data:

• remove unnecessary imports (matplotlib was imported multiple times and skimage's imshow was conflicting with that from pyclesperanto)
• add semi-columns to avoid unnecessary outputs from plots
• change data paths to GitHub urls
• few language improvements