cross border data additions

[smnorris]

• add major channels downstream of BC for visualization and fish passage analysis (ie Chief Joseph/Grand Coulee dams)
• add watersheds that contribute to BC streams for upstream_area calculations

[smnorris]

Re 1 - DFO has done this work already for this paper and has kindly provided the resulting dataset - fundamental watersheds plus contributing area upstream, including updated watershed codes. Using this data as a rough addition to this package for calculating area upstream per stream feature (and modelled channel width / discharge derivatives) should be relatively straightforward.