Divide by zero warnings in costs.py

Occurs when running integration or regression tests.

```
process/costs.py:3028: RuntimeWarning: invalid value encountered in sqrt
    annoam = cost_variables.ucoam[cost_variables.lsa - 1] * np.sqrt(
```

```
process/costs.py:3099: RuntimeWarning: invalid value encountered in sqrt
    annwst = cost_variables.ucwst[cost_variables.lsa - 1] * np.sqrt(
```