DetectorAssembler: assemble with extra shape (multiple frames)

I see that EXtra-geom generally plays well with arrays having some extra dimensions up front. So by conforming to this pattern in terms of buffers, we can pretty easily enable DetectorAssembler to assemble multi-frame images. This could be relevant for:

• Providing a full-detector, full-train, assembled output
  • Has been asked for by different people at different instruments
  • Pretty CPU slow (performance can definitely be improved, this is very naive)
  • Very network slow (that is inherent to desire for full detector on single node)
• Providing multi-frame assembled preview in GUI
  • Karabo GUI will in this case provide a cool slider
  • Should obviously not try to update this at full 2 Hz for network and usability reasons
  • Nevertheless, Jola would be happy to have this option for commissioning and it sort of comes for free in this case