circle.xml: Added the functionality of circle2; the user can define a circle...

circle.xml: Added the functionality of circle2; the user can define a circle by three points on its edge instead of the center and radius. Fixes #518 

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4475 41f8697f-d340-4b68-9986-7bafba869bb8