replace cameraman image due to an uncertain rights

[grlee77]

See scikit-image/scikit-image#3927 on the uncertain rights to this image. We may need to replace it with a suitably licensed alternative.

It looks like our copy was obtained from scikit-image (added in #144)

[rgommers]

Oh fun. I agree, better to replace it even if in practice no one is going to complain.

[grlee77]

A big thanks to @stefanv for following up on this with MIT. It appears it should be okay to keep using it! (reference: scikit-image/scikit-image#3927 (comment))

[grlee77]

Reopening as MIT eventually clarified that the cameraman image as returned by pywt.data.camera() requires a "non-commercial" clause (see scikit-image/scikit-image#3927 (comment)). I will open a PR to replace it with a CC0 licensed variant as in scikit-image/scikit-image#4913