minor updates to checkpoint modifier code to remove hard-coded parameter IDs
minor updates to checkpoint modifier code to remove hard-coded parameter IDs

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