Fix default context in domains

The default context should not be singleton, it is preferable for it to
be picked up automatically (otherwise we would have to always set it
explicitely)

src/shared_utilities/Domains.jl

@@ -228,7 +228,7 @@ end
         periodic::Tuple{Bool,Bool},
         npolynomial::Int,
         longlat = nothing,
-        comms_ctx = ClimaComms.SingletonCommsContext(),
+        comms_ctx = ClimaComms.context(),
         radius_earth = FT(6.378e6)
         ) where {FT}
 
@@ -251,7 +251,7 @@ function Plane(;
     periodic::Tuple{Bool, Bool} = isnothing(longlat) ? (true, true) :
                                   (false, false),
     npolynomial::Int,
-    comms_ctx = ClimaComms.SingletonCommsContext(),
+    comms_ctx = ClimaComms.context(),
     radius_earth = 6.378e6,
 ) where {FT}
     if isnothing(longlat)
@@ -548,7 +548,7 @@ function SphericalShell(;
     nelements::Tuple{Int, Int},
     npolynomial::Int,
     dz_tuple::Union{Tuple{FT, FT}, Nothing} = nothing,
-    comms_ctx = ClimaComms.SingletonCommsContext(),
+    comms_ctx = ClimaComms.context(),
 ) where {FT}
     @assert 0 < radius
     @assert 0 < depth
@@ -635,15 +635,15 @@ end
         radius::FT,
         nelements::Int
         npolynomial::Int,
-        comms_ctx = ClimaComms.SingletonCommsContext(),
+        comms_ctx = ClimaComms.context(),
     ) where {FT}
 Outer constructor for the `SphericalSurface` domain, using keyword arguments.
 """
 function SphericalSurface(;
     radius::FT,
     nelements::Int,
     npolynomial::Int,
-    comms_ctx = ClimaComms.SingletonCommsContext(),
+    comms_ctx = ClimaComms.context(),
 ) where {FT}
     @assert 0 < radius
     horzdomain = ClimaCore.Domains.SphereDomain(radius)