Verifies that an interval definition is valid for a PKNCAdata object.
Valid means that intervals are a data.frame (or data.frame-like object),
that the column names are either the groupings of the PKNCAconc part of
the PKNCAdata object or that they are one of the NCA parameters allowed
(i.e. names(get.interval.cols())).
The column that the impute setting of the PKNCAdata object names (see
get_impute_method()) is also allowed, and it must be a character column,
or a pknca_error_interval_impute_not_character error is raised.
An interval_id identifies one interval: rows that share it may differ
only in the parameters they request (and impute), not in start, end,
or the groups, or a pknca_error_secondary_id_conflict error is raised.
It will return the intervals argument unchanged, or it will raise an error.