add_concept assumes that we are dealing with a ts_tbl

The `add_concept()` function calls `interval(x)` on its input table `x`. This does not work when `x` is an `id_tbl`. See [corresponding line](https://github.com/eth-mds/ricu/blob/c1206dabedd6655bb98ab9fd2a4ba24b98f4aa37/R/callback-itm.R#L344).