Respect layer-bbox when handling wkw files

[jstriebel]

We should respect the layer's bounding box when handling wkw files explicitly in the (mag)view code, e.g.

• MagView.get_bounding_boxes_on_disk() and MagView.get_views_on_disk() should get a respect_layer_bounding_box=True argument, which by default only yields the actual bboxes.
• dataset upload should also only include wkw files within the bbox
• …?

Also, we should enable to remove wkw files outside the bbox, e.g.

• have a method to remove all wkw-files outside the layer bounding box
• warn when there are wkw-files outside the layer bbox, (on initialization and when shrinking the layer-bbox)
• we could also consider removing files when large parts are written as black, but that's probably a made-up edge-case