Divide by zero warnings in availability.py

Occurs when running integration or regression tests.

```
process/availability.py:149: RuntimeWarning: divide by zero encountered in scalar divide
    cv.bktcycles = (fwbsv.life_blkt_fpy / pulse_fpy) + 1
```

```
process/availability.py:189: RuntimeWarning: divide by zero encountered in divide
    tv.t_plant_pulse_burn / tv.t_plant_pulse_total
```