`insert_atomic_to_complex` and `insert_struct_to_complex` do not check if the replaced agent is unspecified (i.e. atomic has `'_'` feature and structure has no atomics specified)

[xtrojak]

If we are trying to replace atomic/structure but there is no available candidate, we should rise an exception an notify the user.

[xtrojak]

insert_atomic_to_struct is also not checking any corner cases.