OverflowError: cannot convert float infinity to integer

using the `case9241pegase.m` from [matpower8.1/data](https://github.com/MATPOWER/matpower/tree/master/data)

```python
from matpowercaseframes import CaseFrames
cf = CaseFrames('case9241pegase.m')
```
results in `OverflowError: cannot convert float infinity to integer` as there are Inf and -Inf values in the file.