updates BigFastTreeModel to work with checkpointing
updates BigFastTreeModel to work with checkpointing

adamallo/beast-mcmc,beast-dev/beast-mcmc,adamallo/beast-mcmc,beast-dev/beast-mcmc,beast-dev/beast-mcmc,adamallo/beast-mcmc,adamallo/beast-mcmc,adamallo/beast-mcmc,beast-dev/beast-mcmc,beast-dev/beast-mcmc,beast-dev/beast-mcmc,adamallo/beast-mcmc