MCMCMC mechanism for creating multiple chains duplicates the entire model

[tgvaughan]

This makes it impossible to use references to BEASTObjects as keys, even when those BEASTObjects represent things like a Taxon or Locus.

Maybe instead of serializing, regexing and deserializing MCMCMC could work entirely with the BEASTObjects themselves. Only CalculationNodes and StateNodes need to be copied after all...