Pytest System Convergence

Hiya,

Can you explain the system model that you use for [the convergence test](https://github.com/m-lundberg/simple-pid/blob/master/tests/test_pid.py)? I don't really understand what this system is supposed to represent.

```python

    def update_system(c, dt):
        # Calculate a simple system model
        return pv + c * dt - 1 * dt
```

Thanks!

