Fix for q_recomb returning NAN

q_recomb changed from 

```
 2.07e-16 / (T**3/2) * exp (h nu / kT) * gl/gu * q_ioniz
```

to the equivalent

```
3.2085e-3 * gl / gu  / electron_temperature * gaunt * cont_ptr->x[0] * (k / h nu)
```

to stop the exponential blowing up at low temperatures