adds date uncertainty to data model.
adds date uncertainty to data model.

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