Given that the MCMC sampler only stores present infections (ie. there are no entries for 0s from the infection history matrix), for some summaries we need to add these 0s back in to avoid bias.
pad_inf_chain(inf_chain, pad_by_group = FALSE, times = NULL, indivs = NULL)the data table with infection history samples from serosolver
if TRUE, accounts for population group when expanding
if not NULL, uses this as a vector of times to replace j when expanding to all combinations
if not NULL, uses this as a vector of individuals to replace i when expanding to all combinations
the same `inf_chain` with 0s added for missing `i`/`j`/`samp_no` combinations, and `population_group` combinations when `pad_by_group = TRUE`