[BREAKING] Multi-channel (multi-input, multi-output) models

[sdatkinson]

It would be nice for models to be defined as having an arbitrary number of channels as inputs/outputs.

One thing that this would allow is for "(directed, acyclic) graph" NAMs to be defined beyond the "serial" definition in #142 so that one can implement "fan-out" / "fan-in" "models" that can be used for parallel processing (even though a "serial" model is already really useful...I just want this too!)

This would end up changing a few things about models that need to be thought through. Looking at the abstract interface in dsp.h:

• Everything about I/O calibration and loudness needs to be multi-channel:
  • Setters and getters, "has X?"
• It's conceivable but maybe not so useful to have different sample rates per channel (obviously no one else does this, but that's not a good reason not to at least think about it).
• And obviously the process() function needs double pointers.