silx view: For NXdata with 3D dataset, avoid loading the whole dataset by default

[t20100]

As it is a NXdata group with a 3D dataset are opened differently depending on the presence of a @interpretation = "image" attribute:

• With @interpretation = "image": A view displaying one slice is used and slicing is only possible along one dimension
• Without @interpretation = "image": A StackView is used which allows to slice the stack in all 3D dimensions but also loads the whole stack in memory.

IMO, it would be best to use the same widget no matter of @interpretation = "image".

Anyway, the whole image stack should not be loaded in memory at first by default which is the case not without @interpretation = "image". The StackView should not load the whole data but only the displayed slice... or another view should be used (at the very least by default).