ZoomTool.h: 2010-07-14 Jeannie Walldren - Fixed bug that caused 1:1 to fail....

ZoomTool.h: 2010-07-14 Jeannie Walldren - Fixed bug that caused 1:1 to fail. Added iExceptions if value is manually entered in text box is invalid. Updated documentation.  

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@180 41f8697f-d340-4b68-9986-7bafba869bb8