ScanR: fix exposure time units (rebased onto dev_5_0)



This is the same as gh-1187 but rebased onto dev_5_0.

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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.