Fix definition of MCMC IDs: currently they are not unique within MCMC tags

[arob5]

MCMC IDs are currently unique by MCMC tag/em tag.

• Fix to make them unique within MCMC tag.
• Update affected code: get_mcmc_ids(), get_ids_by_tag(), get_mcmc_stats_agg()
  Note that I am not actually currently using MCMC tags/IDs for merging variables in get_ids_by_tag(), but I likely will have to when accounting for sequential design rounds. In rounds 2 and after, the design IDs are functions of MCMC IDs/tags from previous rounds, which is not currently accounted for in this function. Need to make sure these previous round IDs are not confused with the MCMC IDs that will be created in the current round.