Add ability to save sediment discharge

[elbeejay]

We calculate sediment flux, qs, but we provide no options for saving this output.

We should add options to save the figures and grids for this variable.

[amoodie]

yep, I agree.

I think an overhaul of the netCDF file, in general, is needed.

For example, I think we should save boundary conditions, too. This relates to things we've discussed elsewhere (but I can't find at the moment).

Thanks for bringing this to the front with an issue. I'm going to tackle this soon.

[amoodie]

Here is an example of the structure we landed on in some old discussion on a slack thread.

output.ext
|  elevation: 
|    | data: t-x-y array
|    | meta: t-value array, keys linking to main meta...
|  velocity: t-x-y array
|    | data: t-x-y array
|    | meta: t-value array, keys linking to main meta...
|  meta: a main metadata set, with some keys linking whatever, maybe containing grid coords?

I'll use this as a starting point for reworking the netcdf file.