Divide by zero warnings in superconducting_tf_coil.py

Occurs when running integration or regression tests.

```
process/superconducting_tf_coil.py:2096: RuntimeWarning: divide by zero encountered in scalar divide
    tfcoil_variables.n_tf_coils
```

```
process/superconducting_tf_coil.py:2109: RuntimeWarning: divide by zero encountered in scalar divide
    tfcoil_variables.n_tf_coils
```