Don't zoom into images

[sschuhmann]

Images are currently scaled to the size of the view. Small images are therefore zoomed in, leading to pixelated output.

In a later revision, images should not get scaled to a scaling factor > 1

[sschuhmann]

Comment from @RaphaelArkadyMeyerNYU
Images output by cells seem to always use the entire width of the window. This means that, on a landscape screen, a square plot does not fit on my monitor. This makes working with plots utterly unreasonable.

As an relatively quick fix, I suggest that we should be able to specify either

A fraction output_image_relative_width (default value: 0.33) in the settings such that the output images have width = output_image_relative_width * window_width OR
A fraction output_image_width (default value: 500) in the settings such that the output images have width = output_image_width

I work in Julia, which produced the following output with the pyplot library that appeals to matplotlib:

[sschuhmann]

@ConstantinGahr added a PR to scale output images to the window size if they are bigger than the current width of the view, but don't zoom them, if they are smaller.

From this PR I currently see the possible implementations:

1. Don't scale images if they are smaller, but always fit to screen width if they are bigger, as default behavior
2. Make this configurable with a setting (zoom to width)
3. Introduce a scale factor as proposed by @raphael

[sschuhmann]

#121 PR with new scaling behavior