standardize the spp upstr/dnstr columns in crossings and streams tables?

[smnorris]

• source for crossings columns is bcfishobs.fiss_fish_obsrvtn_events (all species)
• source for streams columns is bcfishpass.observations (all target species)

questions/todo:

• why not list all species in the streams table, as in the crossings table?
• calculate the crossings columns by inserting to lookup table as per streams for speed/simplicity rather than running slow updates

Note that both these sets of columns are separate from access models - they use bcfishpass.observations and filter the included species (and observation date) as appropriate for the given model (for removing natural barriers downstream of observations).