Function identify_as_tonic_or_dominant 

pub fn identify_as_tonic_or_dominant(
    chord: &Chord,
    key: &Key,
) -> Result<Option<String>>

music21’s identifyAsTonicOrDominant, so G B D F in C major is V7 and A C E in A minor is i. A chord containing the tonic is the tonic, one containing the dominant is the dominant, and anything else is judged by how many of its notes the tonic and dominant sevenths share; None when neither wins.