Adjust contrast of image

[lars-reimann]

Is your feature request related to a problem?

I'd like to be able to adjust the contrast of an image.

Desired solution

Add a new method adjust_contrast to Image that takes one parameter factor and returns the adjusted image. The original should be left unchanged.

Regarding factor:

• A factor in (1, inf) should increase the contrast of the image
• A factor in [0, 1) should make the image more gray
• A factor of 1 should keep the image unchanged

Because of this:

• Raise an error if the factor is < 0.
• Warn if the factor is equal to 1.

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