Split reported case integer time series into train and validate time series through thinning.
train_val_split(reported, frac_train = 0.75)An integer vector of reported cases.
A numeric between 0 and 1 for fraction of data used to train lambda validation.
A list(reported_train, reported_val) where the elements reported_train and reported_val are both length, Tobs, and 'frac_train' of the counts fall in reported_train, the rest in reported_val.