Change aggregation from mean to max for nwm_agg

[quinnylee]

Closes #119

Changes

• Changes restart calculation from mean to max
• See issue for scientific reasoning

Testing

1. Tested locally
2. Some metrics

Mean-based calculation vs USGS observation (all gages | excluded outlier gages)

• MAE: 14.56 | 7.18
• RMSE: 224.23 | 58.03
• r: 0.785 | 0.978

Max-based calculation vs USGS observation (all gages | excluded outlier gages)

• MAE: 12.55 | 5.14
• RMSE: 223.23 | 54.63
• r: 0.787 | 0.981

Notes

• Definitely not a drastic change, but is a slight improvement until a more robust mapping is developed