feat: add computation of ROI mean values in stack

[EscapedGibbon]

close: #467

[targos]

This is too specific for a public API, with too many hidden things happening inside.

Maybe this should be a tutorial in the documentation?

[EscapedGibbon]

This is too specific for a public API, with too many hidden things happening inside.

Maybe this should be a tutorial in the documentation?

I think it is too specific as a standalone tutorial as well. We need also to add some documentation about how to work with Stack and what it is in general. But I think it can be a tutorial indeed. @stropitek @lpatiny WDYT?

[stropitek]

I agree it is way too specific to be implemented directyl into image-js.

I supppose you tested it on images with a real use case so it would be interesting to have a tutorial with the results in image-js-docs