Added a multi-region tracer advection test case

It is not working, most probably due to an error associated with
PrescribedVelocityFields on the conformal cubed sphere grid.

Author: siddharthabishnu

validation/multi_region/multi_region_tracer_advection.jl

@@ -0,0 +1,87 @@
+using Oceananigans
+using Oceananigans.MultiRegion: getregion
+using Oceananigans.Utils: Iterate,
+                          get_lat_lon_nodes_and_vertices,
+                          get_cartesian_nodes_and_vertices
+using Oceananigans.BoundaryConditions: fill_halo_regions!
+
+
+function multi_region_tracer_advection!(Nx, Ny, Nt, tracer_field)
+
+    grid = ConformalCubedSphereGrid(panel_size=(Nx, Ny, Nz = 1), z = (-1, 0), radius=1, horizontal_direction_halo = 1, 
+                                    partition = CubedSpherePartition(; R = 1))
+    
+    u0 = Oceananigans.Fields.ZeroField()
+    v0 = Oceananigans.Fields.ZeroField()
+    
+    velocities = PrescribedVelocityFields(; u = u0, v = v0)
+
+    model = HydrostaticFreeSurfaceModel(; grid, velocities, tracers = :θ, buoyancy = nothing)
+
+    θᵢ(x, y, z) = exp(-x^2/100 - y^2/100)
+    set!(model, θ = θᵢ)
+
+    fill_halo_regions!(model.tracers.θ)
+    
+    Δt = 1.0
+    T = Nt * Δt
+    
+    simulation = Simulation(model, Δt=Δt, stop_time=T)
+    
+    # Figure out OutputWriters for a CubedSphere (`reconstruct_global_grid` is not defined for a CubedSphere neither 
+    # does it make sense to define it for a CubedSphere). Then we can use the following lines of code:
+    # 
+    # simulation.output_writers[:fields] = JLD2OutputWriter(model, model.tracers, schedule = TimeInterval(0.02),
+    #                                                       filename = "tracer_advection_over_bump")
+    #                                                                             
+    # simulation.output_writers[:fields] = Checkpointer(model, schedule = TimeInterval(1.0), 
+    #                                                   prefix = "multi_region_tracer_advection")
+
+    function save_tracer(sim)
+        push!(tracer_field, sim.model.tracers.θ)
+    end
+
+    simulation.callbacks[:save_tracer] = Callback(save_tracer, TimeInterval(1.0))
+
+    run!(simulation)
+    
+    return nothing
+end 
+
+
+Nx = 10
+Ny = 10
+Nt = 10
+
+tracer_fields = Field[]
+multi_region_tracer_advection!(Nx, Ny, Nt, tracer_fields)
+tracer = tracer_fields[1]
+
+@info "Making an animation from the saved data..."
+
+n = Observable(1)
+
+θGrid = @lift begin
+    interior(tracer_fields[$n], :, :, 1)
+end
+
+fig = Figure(resolution = (850, 750))
+title_θ = @lift "Tracer Concentration after " *string(round(times[$n], digits = 1))*" seconds"
+
+@apply_regionally heatsphere!(ax, θGrid)
+
+ax_θ = Axis3(fig[1,1]; xlabel = "Longitude (m)", ylabel = "Meridional Distance (m)", xlabelsize = 22.5, ylabelsize = 22.5, 
+             xticklabelsize = 17.5, yticklabelsize = 17.5, xlabelpadding = 10, ylabelpadding = 10, aspect = 1.0, 
+             title = title_θ, titlesize = 27.5, titlegap = 15, titlefont = :bold)
+
+@apply_regionally heatsphere!(ax_θ, model.tracers.θ)
+
+Colorbar(fig[1,2], hm_θ; label = "Tracer Concentration", labelsize = 22.5, labelpadding = 10.0, ticksize = 17.5)
+
+frames = 1:length(times)
+
+CairoMakie.record(fig, filename * "_" * immersed_boundary_type * "_" * plot_type * ".mp4", frames, framerate = 8) do i
+    msg = string("Plotting frame ", i, " of ", frames[end])
+    print(msg * " \r")
+    n[] = i
+end