ImageJ: enable min/max calculation if any series contains float data

Fixes the bug noted in https://github.com/openmicroscopy/bioformats/pull/1480#issuecomment-66973008.  The issue only appeared when the ```Autoscale``` option is turned off, the first image does not contain floating point data, and a subsequent image does contain floating point data and is the one selected to be opened.  

With this change, repeating the test from gh-1480 should result in the image being opened without an exception.

/cc @pwalczysko