ND2: ignore all blocks of image offsets except for the last one

Extra, possibly invalid, planes are sometimes stored before the first
true image in the dataset.  We now do what NIS Elements does, and ignore
everything except for the last continuous block of image offsets.

See
http://lists.openmicroscopy.org.uk/pipermail/ome-users/2014-July/004589.html
ND2: adjust default buffer size to speed up initialization

See http://trello.com/c/1ZClsqPU/42-nd2-performance
Merge pull request #1229 from melissalinkert/rebased/develop/nd2-final

Final .nd2 fixes for 5.0.3 (rebased onto develop)