generated network_edges (CostGraph) file has columns with multiple data types

This wasn't seen in the glass study and may or may not be an actual problem.

```
loading routing graph
C:\Users\rhanes\GitHub\celavi\celavi\routing.py:200: DtypeWarning: Columns (4,5,19,20) have mixed types. Specify dtype option on import or set low_memory=False.
  network = pd.read_csv(network_edges)
```