Implement PLC

[gavv]

• [x] Finish dependency tasks

  • 615 - allow reporting partial read and teach readers to handle it

    • needs #614 - teach readers reporting status codes
    • needs #697 - return frame instead of filling pre-allocated frame, so that reader can define frame size, not caller

  • 730 - allow reading signal and gaps separately (using partial reads mechanism)

    • needs #304 - inspect next packet without dequeuing it

• [x] Update Depacketizer

  • [x] generate partial reads to distinguish gaps and signal
  • [x] handle soft reads
  • [x] forward errors and StatusDrain from packet reader
  • [x] do peek read before fetch read, and don't do a fetch if it's not needed yet (will fix #54 and #210)
  • [x] unit tests

• [ ] Implement PLC readers

  • [x] ProcessorMap
  • [ ] PlcBackend, IPlc
  • [ ] PlcReader
  • [ ] BeepPlc
  • [ ] LinearPlc
  • [ ] PluginPlc
  • [ ] unit tests
  • [ ] integrate into pipeline

• [ ] Pipeline tests

  • [x] partial reads
  • [x] soft reads
  • [x] big reads
  • [ ] plc

• [ ] CLI

  • [ ] --plc=none,echo,beep
  • [ ] docs

• [ ] Add PLC support to public API

  • [ ] add roc_plc_backend (ROC_PLC_DISABLE, ROC_PLC_DEFAULT, ROC_PLC_LINEAR, ROC_PLC_BEEP)
  • [ ] add plc_backend field to roc_receiver_config
  • [ ] pass from API to pipeline
  • [ ] add roc_context_register_plc(roc_plc_backend, func_table)