provide a Onda.AbstractLPCMFormat subtype

[jrevels]

We didn't take this path originally because Beacon has little interest in EDF as a long-term storage format, but technically, the "right" approach for this package would be to implement (de)serialization to/from EDF from/to Onda's canonical in-memory LPCM representation, then any conversion utilities would be simple wrappers around that (and wouldn't even be necessary for loading the file, e.g. you could just reference EDF sample data artifacts directly).