Determine and update the chain statistic being tracked
.update_chain_stat(stat_type, stat_latest, n_offspring)A vector of chain statistics (size/length). A numeric vector coercible to integer.
Chain statistic (size/length) to update; A character string. Must be one of 'size' or 'length'.
The latest chain statistic numeric vector to be updated.
A vector of offspring per chain. A numeric vector (coercible to integer).