sub models don't cover upstream ends of first order tributaries

[sclaw]

When a source model fully covers a first-order NWM reach, the generated sub model does not include cross-sections upstream of the NWM reach limit. This leads to incomplete FIM coverage.

Collection: mip_17090003
Model: uw_group27
Sub model: 23765425

Black pixels are FIM, but they stop at RS 13309. RS 14277 should be included in sub model to provide full FIM coverage.

Suggested fix: when a NWM reach has no NWM reaches upstream of it, add logic to make us_xs one section higher.