WHY IS THIS CHANGE NEEDED?:
- when tiler.initialise_traps() is invoked, "No valid tiling regions
  found" is raised from traps.segment_traps() (aliby.tile.traps).

HOW DOES THE CHANGE SOLVE THE PROBLEM?:
- the image called by segment_traps() was a blank image.
- this was because tiler.initialise_traps() uses the first timepoint to
  define the initial image (line 400 in tiler.py).
- the first timepoint was a blank image because ImageDummy.pad_array()
  added zero arrays BEFORE the input array.
- so fixed it by reversing the behaviour of ImageDummy.pad_array()
- and modified the test_image test to reflect this change.

WHAT SIDE EFFECTS DOES THIS CHANGE HAVE?:
- none known -- only potential effect is the test, which was modified

EVIDENCE THAT COMMIT WORKS:
- test_image passed