ScanR: fix exposure time units

See http://www.openmicroscopy.org/community/viewtopic.php?f=13&t=7554.

To test, verify that exposure times are smaller with this change.  I was using ```data/from_skyking/scanr/nadine``` to test, where the original exposure time was ```50.0``` and the new time is ```0.05```.  Other datasets should produce similar results.