Add in error or warning if region and time_point not complete

[sempwn]

Unhelpful error generated when using data that has missing region and time_point pairs

fit <- bennu::est_naloxone(THN_data)
Error in FUN(X[[i]], ...) : Stan does not support NA (in Orders) in data
failed to preprocess the data; sampling not done

Should have some data check before model fitting to ensure that data formatted correctly. Question whether better to do this by filling in missing values or by generating error and allowing user to check their data first.