make the merge grid transformation work for large images

[tischi]

Currently the GridSourceTransformer crashes when the individual images are too large, because then the cells become bigger than the java indexing allows.

Maybe a solution could be to have one cell per z-plane.

[tischi]

@constantinpape @martinschorb

I think for now we should do the following:

• less than 100 big images => transformGrid
• more than 100 small images => grid
• anything else => discuss with Tischi

The issue is that when we are using the current grid option we are essentially loosing all the lazy loading of the individual sources, but they are completely loaded into RAM! I think I could work on this, but it will likely depend on the use case, so we should discuss it case by case.

[constantinpape]

Ok!

@martinschorb see this example for how to choose between grid and transformGrid.