Add a symbol that codes for `{A,C,G,T}`

[Taepper]

With #1201, N simply means missing information -> also a gap symbol - can be at that position.

Therefore, there is no symbol that codes for the set {A,C,G,T}.

This should be added so there is an ambiguous symbol for every set S, where S ⊆ {A,C,G,T}