Can't find exec.py

Hello,

I followed the instructions in the README.md on github:

```
git clone https://github.com/DeepLearnPhysics/larcv3.git
cd larcv3
git submodule update --init
pip install -e .

Then, in the CosmicTagger directory,

pip install -r requirements.txt
```

Everything seemed to go fine - no error messages. However, I can't find exec.py. Is that created during the install? 

Thanks!
