Remove either frames property or __len__() from SoundFile

There should be only one obvious way to get the length of a sound file (according to the Zen of Python).

The question is, which one should we keep?

`len()` is idiomatic for getting the length of something, but `frames` is more explicit because it avoids misunderstandings regarding frames vs. samples vs. bytes.
`frames` is used multiple times as a function argument, but I don't know if this counts as an advantage or disadvantage for the `frames` property.