Rename divisions to something that makes more sense

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The idea of "divisions" was lifted from the Tcl/Tk FreightCarForwarder program, 
and gave a way to decide which cars were suitable for which industries, and 
where those cars would exit.  That concept's a little awkward and hard to 
explain; the main reason we use divisions now is for determining the exit 
routes for cars.

Choose a different name for divisions to match this concept, and redo code and 
docs to match.

Original issue reported on code.google.com by rwbowdi...@gmail.com on 14 Mar 2012 at 11:30