S3 generic for removing objects. The TrialComparison method
delegates to $delete() and modifies the existing comparison by reference.
The TrialComparison method deletes one contained object by position or name.
Examples
comparison <- getComparison(fixed = getDesignFixed(),
sequential = getDesignGroupSequential(kMax = 2))
comparison |> delete("fixed")
comparison |> delete(index = 1)
comparison |> add(getDesignFixed()) |> delete(1)
