Investigate making Scale and Chord generic over their note type

[fwcd]

This would e.g. enable forming scales and chords over both Note and NoteClasses. Perhaps we could do this using a new protocol:

protocol DiatonicIntervalArithmetic {
  static func +(lhs: Self, rhs: DiatonicInterval) -> Self

  static func -(lhs: Self, rhs: DiatonicInterval) -> Self

  static func +=(lhs: inout Self, rhs: DiatonicInterval) -> Self

  static func -=(lhs: inout Self, rhs: DiatonicInterval) -> Self
}

extension Note: DiatonicIntervalArithmetic {}

extension NoteClass: DiatonicIntervalArithmetic {}

and then rewrite the protocols e.g. as follows

protocol Scale {
  associatedtype Note: DiatonicIntervalArithmetic

  var notes: [Note] { get }
}

protocol Chord {
  associatedtype Note: DiatonicIntervalArithmetic

  var notes: [Note] { get }
}

(though we may want to name the associated types differently to avoid confusion with the Note struct).