Phase 4: wire MIR/PMI into the AMICA API

[neuromechanist]

Part of #133. Depends on Phases 1-3.

Expose model.mir(X) / model.pmi() on AMICA and AMICATorchNG, computable
at fit-end and optionally at specified iteration waypoints, per Scott Makeig's
original request (applicable "at decomposition end and/or optionally at
specified decomposition waypoints").