consistent samplerate API

[ssfrr]

Throughout a bunch of the audio code I write samplerates are not handled consistently. For instance the SampleBuf constructor takes the samplerate as a required argument, but in many other places where I want code that can also accept regular Arrays I usually include samplerate as an optional keyword argument that affects how any frequency arguments are interpreted. I have a bunch of notes with pros and cons of various APIs for handling samplerates - I need to pick one and use it consistently.

[bramtayl]

Maybe samplerate should return a unitful quantity too?