Call to run demo fails as pt file not included in package

Running
```
pip3 install nasim
pip3 install nasim[dqn]
python3 -m nasim.demo tiny -ai
```
fails as required file `dqn_tiny.pt` not included as part of package during pip install

```bash
FileNotFoundError: [Errno 2] No such file or directory: '/usr/local/lib/python3.9/site-packages/nasim/agents/policies/dqn_tiny.pt'
```