Simplify the interface

[briochemc]

Maybe the interface should be in a single call that dispatches and does all the work

T = transportmatrix(; 
    umo_ds, # or uo_ds
    vmo_ds, # or vo_ds
    areacello_ds,
    volcello_ds,
    mlotst_ds,
    ρ, # <- maybe replace with S and T if generally available?
    κH,
    κVML,
    κVdeep
)

To dispatch on whether the fluxes or velocities are provided, I think maybe I could use a pattern like

transportmatrix(; uo=[], vo=[], umo=uo2umo(uo), vmo=vo2vmo(vo) = ...

but I'm not sure this is the Julian way?