Divide by zero warnings in fusion_reactions.py

Occurs when running integration or regression tests.

```
process/fusion_reactions.py:769: RuntimeWarning: divide by zero encountered in divide
    xi = ((reaction_constants.bg**2) / (4.0 * theta)) ** (1 / 3)
```

```
process/fusion_reactions.py:774: RuntimeWarning: invalid value encountered in multiply
    1.0e-6
```