Wrong test in main.py - dipole_field_test():

In the function dipole_field_test() of main.py, the fourth test comment and actual test do not match (-3 on comments, 3 on test):
```
  # At (y, z) = (0, -1), the field should be (0, -3)
  assert np.all(dipole_field([-1, 0]) - np.array([0, 3])) < 1e-15
```