Build the diatonic seventh chord rooted on a scale degree.
1-based scale degree of the chord root: 1 is the tonic.
Key/scale context, as a key name, a key/scale, or a Key.
Key
The diatonic seventh chord.
If the scale is not heptatonic, for the reason given on diatonicTriad.
Build the diatonic seventh chord rooted on a scale degree.