added from_param class method to vec2d to allow non Vec2d instances (like (x,y)-tuples) sent to most methods expecting a Vec2d. 
added from_param class method to vec2d to allow non Vec2d instances (like (x,y)-tuples) sent to most methods expecting a Vec2d. 

git-svn-id: 4658502d6a5631d0681d7730b53f9627060b5bd8@123 78f3efd8-5644-0410-8aa8-f3833044bd4a

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