Checks that si is a list with exactly the elements inter, prey, bait,
and that each is a data frame.
Examples
si <- list(
inter = data.frame(ipId = "IP1", baitId = "BaitA", preyId = "Prey1"),
prey = data.frame(preyId = "Prey1"),
bait = data.frame(ipId = "IP1", baitId = "BaitA", CorT = "T")
)
validate_saint_input(si)