Support note tunings

The `Note` object currently supports conversion to Hertz, but no conversion back. We have Hertz values hard-coded, meaning a different tuning isn't possible.

We should allow users to use a new `Tuning` object that specifies how to calculate the hertz value for the notes. We could implement this as a Trait, meaning it could be extended by library users.

ex:
```rust
pub trait Tuning {
fn to_hertz(&self, &Note) -> u32;
fn from_hertz(&self, u32) -> Option;
fn from_hertz_nearest(&self, u32) -> Note;
}
```

Look at the existing Note struct and its to_hertz method. The new Tuning trait should be defined in a module like src/tuning.rs. Implement a default equal-temperament tuning first. Check how Note is constructed and where hertz values are currently hardcoded to replace them with the trait. Write tests for conversion round-trips.

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