better subarray support

[ssfrr]

Currently you can use sub on a SampleBuf, but you lose all the samplerate information, unitful indexing, etc.

It seems like we could tweak SampleBuf to contain any AbstractArray, and add a definition for sub that gives you a new SampleBuf that wraps the view instead of a copy. Something like

sub(buf::SampleBuf, A...) = SampleBuf(sub(buf.data), samplerate(buf)) seems like it should be pretty close.

[haberdashPI]

#44 takes care of much of this. I don't think using view (the new name for sub) works, but that should be easy to implement.