[IO] Improve image stack handling

[vasole]

The code as it was written was only dealing with a single dataset of images.

This PR makes sure if multiple scans and datasets are selected, they are properly summed.

It falls back to the case the data were not fitting into memory that was lost when avoiding a loop.

[woutdenolf]

Oef tricky stuff

[vasole]

Oef tricky stuff

Indeed. Too many possibilities offered to users.

For instance, Canadian Light Source prefers to be able to select multiple datasets per scan to have PyMca perform the addition of separate channels of a multielement detector.