Update autocuts algorithms

[ejeschke]

This PR updates the auto cuts algorithms in the following ways:

• auto cuts features in the viewer can now work properly with images plotted on the canvas (not just a single loaded image); for example, the output of the Collage plugin
• the auto cuts algorithms that defaulted to taking a small crop sample from the center of the image ('histogram' and 'stddev') are expanded to include an option to sample the data in a grid pattern across the image. This is useful for large images (collages or large data frames such as mosaics), where a small crop from the center is not representative of the data across the image and the image is much too large to apply the algorithm to the full data.
• adds a full_px_limit parameter to 'histogram' and 'stddev' that specifies the number of pixels limit (in image size), beyond which the full image data calculation falls back to a crop image calculation. This prevents the viewer from accidentally attempting to calculate cut levels on a really large image, which can lock up the viewer.

Here is an example of the PR improvement. First, a collage using the 'zscale' auto cuts (before PR)

and after PR:

The documentation has been greatly enhanced in the AutoCuts module and the user manual will be expanded to include it.

[ejeschke]

Hi @pllim, have a look when you get a chance. No hurry.

[ejeschke]

Rebased

[ejeschke]

Out of scope here but I would still like #560 to happen someday...

We'll have to put together a sprint for that one, I think.

[ejeschke]

Thanks for the review, @pllim! I added a comment to the change log that will hopefully clarify the change for stddev and histogram algorithms (the only ones affected).