ability to edit pixels of multilayer exr image datablocks via python

It's often necessary to be able to modify, overwrite and create compressed multilayer exr files from python.

Currently this isn't possible as image datablocks that have a mulitlayer exr loaded into them, don't allow access to the contents. All other file types do, jpg, single layer exr, png, etc.

This ability would open up a lot of doors for addon developers, helping to provide end users with all sorts of modern machine learning based tools.