Use cubic hermite interpolation for overdubbed playback

[rconstanzo]

This issue is moved over from issue #5 so that one can be closed by commit 3c6098d.

Summary:
There is some aliasing when playing back audio during overdub mode.

Steps to reproduce:
Create a loop and set speed to something low (0.1), then fill the buffer with vibes-a1.aif. Set overdub param to 0.0. Then send record messages to go into and out of overdub mode.

Actual results:
When playback is normal, interpolation sounds smooth. The moment you go into overdub mode, the playback of the vibes sounds interpolate-y.

Expected results:
It should sound the same.

[rconstanzo]

This may not be worth incorporating but perhaps an interim solution would be chuck a filter on the playback head during ‘overdub’ to avoid the alias-y sound.

There’s likely something else happening here that may be just as easy to fix (once found) rather than adding a filter in, but just putting the idea here to be thorough.