Fix silent error in `geometry/huc12_reach_analysis.ipynb`

[xaviernogueira]

Upon learning that HUC12 basins can have multiple COMID outflows, despite having a 1-to-1 outflow relationship with another HUC12.

This means that huc12_reach_analysis.ipynb, which assumes each HUC12 has just one outflow COMID, is silently overwriting any other outflow COMIDs and wrongly populating the "inflow_comids" column of the final dataframe.