check_input_vec is a helper function for model_trend,
ind_init, statespace_ed and statespace_ch.
It checks whether the time object is an integer vector. If not (a vector
OR integer) it will return an error message.
check_input_vec(x, vec_name)An R object, which should be a numeric vector.
The name of the input object for the error message.
The function returns the checked input vector unchanged.