Add numeric basin ids to basin paths in svg

[hcorson-dosch-usgs]

In order to more easily join the svg paths to the data in our Javscript code (in order to set up interactions), it would be helpful to have the numeric basin ids from the IWS data added to the basin paths in the svg. One potentially tricky bit is that some IWS basins seem to have >1 basin id associated with them (e.g. upper colorado, illinois river basin), so we might need to figure out a clever way to set up the id or class names such that we can parse that information if we want the interaction to be triggered for the IWS basin as a whole. Although, if we end up adding all basins from the dataset to the map, and setting up interactions for each of those basins (rather than only the IWS basins), then I think we'd be back to a 1:1 mapping, which would solve that issue.