Merge branch 'plc_challenge' into master

dnn/Makefile.am

dnn/README.md

dnn/burg.c

dnn/burg.h

dnn/download_model.sh

dnn/dump_data.c

dnn/freq.c

dnn/freq.h

dnn/include/lpcnet.h

dnn/lpcnet.c

dnn/lpcnet_demo.c

dnn/lpcnet_enc.c

dnn/lpcnet_plc.c

dnn/lpcnet_private.h

dnn/nnet.c

dnn/training_tf2/dump_plc.py

dnn/training_tf2/lpcnet_plc.py

dnn/training_tf2/plc_loader.py

dnn/training_tf2/test_plc.py

dnn/training_tf2/train_plc.py