[21pt] Custom boundary conditions for each model

[LauraKeys-NOAA]

Currently we use a set downstream boundary condition for every model we create that is "Normal depth with slope S=0.005".

Setting this slope value to better match a model's actual slope could yield better results, especially in features where we're seeing our results default to critical depths. A few options to consider in modifying the slope values:

• Use slope value from the parent model
• Calculate slope manually from cross sections in our models
• Use multiple slope values for long rivers
• Use parent model boundary conditions that aren't normal depth (Known WSE is used in some places)

This value gets written to the flow file generated in worker_fim_raster.py as Dn Slope.

• Note that we won't need to worry about this if we switch to running / using the originally-supplied RAS files for an entire parent model! But if we continue doing any model-splitting and writing RAS files from scratch, this issue will still be applicable.