harmonic method
The harmonic at index
from this Frequency, including negative
values as part of the undertone series.
Example:
const Frequency(220).harmonic(1) == const Frequency(440)
const Frequency(880).harmonic(-3) == const Frequency(220)
Note.c.inOctave(1).frequency().harmonic(3).closestPitch()
== Note.e.inOctave(3) - const Cent(14)
Implementation
Frequency harmonic(int index) => Frequency(
index.isNegative ? hertz / (index.abs() + 1) : hertz * (index + 1),
);