Add context to reference gage metadata.

[dblodgett-usgs]

Should add drainage area of the linked catchment, drainage area of the monitoring location, and snap distance from monitoring location to linked hydrolocation.

Should also set unknown drainage area sites not within X tolerance of a flowline to nldi index type of point rather than hydrolocation.

While working through this -- also add comments to code describing specifically what is happening.

[dblodgett-usgs]

Some of this got done previously, but in addition to above, need to track the source of the link to characterize how the hydrologic location was arrived at.