Add generic 'extent' and 'breadth' ? functions

[fryguybob]

(Imported from http://code.google.com/p/diagrams/issues/detail?id=45. Original issue from byor...@gmail.com on September 3, 2011, 04:42:54 PM UTC)

Diagrams.TwoD.Util has function like extentX, extentY, height, and width; there ought to be generic (vector-space polymorphic) variants which take as an extra argument a vector specifying the direction in which to measure the extent/width. Then extentX etc. can just be specialized variants of the more generic function.

[fryguybob]

(Imported. Original comment by byor...@gmail.com on November 1, 2011, 04:03:09 AM UTC)

Actually, a generic 'extent' function is a bit harder; what exactly should it return? A mapping from basis vectors to pairs of distances? I'm not convinced this is useful so I didn't add it. But I did add a generic 'breadth' function.