update code to work with numpy >= 2.0

`np.Inf` alias for `np.inf` has been removed.

In `TreatmentCentreModel`

```python
            # set to a large number so that at least 1 sample taken!
            u = np.Inf
```

needs to be 

```python
            # set to a large number so that at least 1 sample taken!
            u = np.inf
```