CellSens: do not set the resolution count

The images in the .vsi file are not proper sub-resolutions of the images
in the .ets file(s), so let's not tell the reader that they are.  While
the aspect ratio may be the same, the channel counts (and/or Z/T sizes)
are frequently different.

Fixes ticket #10354.
Merge pull request #369 from melissalinkert/ticket-10354

CellSens: do not set the resolution count