* Use a configurable number of PCM buffers in a queue to avoid overflows.
* Add option to configure whether to use pendsv or mp_scheduler for Python callbacks.
* Streaming can be started without a callback and get_buffer() returns a single buffer from the queue.
* Add precomputed LUT for lib OpenPDM stored in flash (saves about 500uS per conversion).
* Overflow detection and configurable abort on overflow.
* Remove fixed DMA channel.