ENH: Added bimodal analysis to float and long volumes for auto-W/L

The bimodal analysis calculating a the auto window/level in a smart way was only applied when integer volumes were loaded. The reason for not supporting float, double, and long volumes is that the vtkImageBimodalAnalysis class handles the input image histogram with direct correspondence between bin index and voxel intensity. By automatically determining the histogram bins (instead of the constant -32K - +32K for the integer volumes), and converting the resulting W/L values back to intensity space, it is possible to use the bimodal filter for other volume types. It seems to work well for volumes with scalar ranges in the (-1000,3000) and (-1,2) too.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25940 3bd1e089-480b-0410-8dfb-8563597acbee