How to specify pixel data as read-only

[darcymason]

From discussion in #717. Being able to specify pixel data as read-only can eliminate the need to copy the array in some cases, which can save memory. For very large images or a large number of images, this could be quite advantageous.

Possibly related to #139 also, because specifying read-only would simplify some issues for memory-mapped files.

We need an option in config, or a flag in Dataset, or (some other solution) for users to be able to specify that pixel data (or perhaps any large data array) should be read-only when converted from its raw value.

[darcymason]

Pushing this back a few versions ... I think the need for this was much reduced by fixing the read-only pixel array problem, and I think using Dataset.decompress helps a little by modifing PixelData in place allowing the original memory to be freed up.