Addresses #313 by adding a more flexible variant of SimpleExtendingTraceTranslator: ExtendingTraceTranslator accepts a bijective trace transform that remaps the choices sampled by the forward proposal distribution, before merging those choices into the extended model via update.

Before I move forward with adding documentation and test cases, I'm wondering if perhaps this should just replace the existing SimpleExtendingTraceTranslator? This does everything that SimpleExtendingTraceTranslator does, and I don't think it adds much of a performance overhead to have the if-else clause in ExtendingTraceTranslator that checks whether a trace transform needs to be applied. For backwards compatibility, I could even make SimpleExtendingTraceTranslator a constructor alias for an ExtendingTraceTranslator with the bijection filled in with nothing.