[Discussion] Separate using original training models

Hi! Spleeter is very useful repository! Thx!

You can refer to [the official spleeter wiki](https://github.com/deezer/spleeter/wiki/2.-Getting-started#train-model) to proceed with the training.
In this issue, I will show you the next step, how I use original models to separate test sounds.

I used the spleeter system for audio source separation of field recordings.
Specifically, I tried to classify them into **3 stems: Human / Bird / Car**.

```fish
# Train
spleeter train -p configs/***_config.json -d
# You may be get ***_model folder including checkpoint / model.ckpt-200000.data-00000-of-00001 / model.ckpt-200000.index / model.ckpt-200000.meta.
```

Move the original models to the pretrained_model folder.
```fish
# Create pretrained_model folder
mkdir pretrained_models
mkdir pretrained_models/3stems
# Move model files
mv ***_model/checkpoint pretrained_models/3stems
mv ***_model/model.ckpt-200000.data-00000-of-00001 pretrained_models/3stems
mv ***_model/model.ckpt-200000.index pretrained_models/3stems
mv ***_model/model.ckpt-200000.meta pretrained_models/3stems
mv ***_model/.probe pretrained_models/3stems
```

Embed [3stems.json](https://github.com/makobouzu/spleeter/blob/master/train/3stems.json) into the spleeter.
```fish
# Move 3stems.json
mv 3stems.json $HOME/.pyenv/versions/3.7.7/lib/python3.7/site-packages/spleeter/resources/
```

You can separate test sounds using original models.
```fish
# Separate
spleeter separate -p spleeter:3stems -o output audio_example.mp3
# You can get human.wav / bird.wav / car.wav
```

---

I couldn't find any information on how to embed it in spleeter or how to use models other than musdb, so I had to share this issue.
For detail -> [makobouzu / spleeter](https://github.com/makobouzu/spleeter)
thank you!!

> [PC Settings]
> Mac OS: Catalina
> fish
> pyenv (pyenv-virtualenv)
> python 3.7.7