FEAT make tp2fd and fd2tp public

[MatthewSZhang]

Use feat_ids + delay_ids to compute Jacobian matrix is easier than poly_ids + time_shift_ids. Although poly_ids + time_shift_ids is natural to narx + term selection, feat_ids + delay_ids is more natural to human users. So we adopt feat_ids + delay_ids to define a NARX object for more general use cases. And pt2fd and fd2pt are convenient functions to convert the two types of ids from each other.