ND2: always set the field index to 2

This should fix plane ordering issues reported in
http://lists.openmicroscopy.org.uk/pipermail/ome-users/2014-March/004287.html
and QA 7983.
Merge pull request #1005 from melissalinkert/rebased/develop/nd2-field-order

ND2: always set the field index to 2 (rebased onto develop)