Cleaning up warnings

[kjkrum]

There are 37 pieces of dead code, 4 unused local vars, and 59 Serializable classes with no serialVersionUID.

All but one of the dead code warnings is in the generated prefuse.data.expression.parser.ExpressionParser. They take the form of 'break' statements after 'return' statements or 'throw' statements after 'return' statements preceded by useless 'if(true)' conditions. I know very little about JavaCC, so I have no idea why it generated all this dead and useless code. I fixed it by hand in this commit: https://github.com/kjkrum/Prefuse/commit/59bb669d695f12c65930c08517bec9564a657a1e

I also added a default serialVersionUID to all Serializable classes: https://github.com/kjkrum/Prefuse/commit/64f2c97fac060af6d81fb41fcf2119791b1d1363

I was more cautious about fixing the unused local vars and the one remaining piece of dead code. I didn't want to just go in and remove them without understanding the author's intent, because the dead code may point to a bug. Only one of the unused local vars seemed to have an obvious intent: https://github.com/kjkrum/Prefuse/commit/b740787e59b0d88d01eab52586e4d8aa8b365b4a

I'd appreciate some help understanding the intention behind the remaining four.

prefuse.data.util.BreadthFirstIterator line 168: The increment part of the 'for' is dead code because the method always returns on the first iteration of loop.

prefuse.data.tree.Tree line 587: 'ncount' is incremented but never used.

prefuse.action.layout.StackedAreaChart line 261: 'sum' is incremented but never used.

prefuse.action.layout.graph.SquarifiedTreeMapLayout line 135: 'leafCount' is incremented but never used.