Numpy deprecation warning

```
hypnotoad/utils/critical.py:186: DeprecationWarning: Conversion of an array with ndim > 0 to a
scalar is deprecated, and will error in future. Ensure you extract a single element from your
array before performing this operation. (Deprecated NumPy 1.25.)
    J[1, 0] = -Bz / R1 + f(R1, Z1, dx=2) / R1
```

Not sure which bit is the issue here