Continued rewriting the interval exchange classes and Mod1Number
classes. As now I see it, I should probably merge the
IntervalExchangeMap and TwistedIntervalExchangeMap classes, since the
first is actually an easy special case of the second. I probably don't
even need the IntervalExchangeMap class anyways, but even if I do, I can
easily derive it from TwistedIntervalExchangeMap.