Restore Fields.ColumnField as an alias of FiniteDifferenceField

The rewrite removed ColumnField (a Field with DataColumn values), but
ClimaAtmos main uses it as a type annotation, so ClimaAtmos and
everything depending on it (ClimaCoupler) fail to precompile against
this branch. Single-column fields are now identified by their space, so
the old ColumnField is the same set of fields as FiniteDifferenceField:
FD fields and column slices of extruded fields match the alias, full
3-D fields do not. Verified that ClimaAtmos main precompiles and loads
against a patched worktree.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: dennisYatunin

src/Fields/Fields.jl

@@ -64,6 +64,12 @@ const SpectralElementField2D{V, S} =
 
 const FiniteDifferenceField{V, S} =
     Field{V, S} where {V <: DataLayout, S <: Spaces.FiniteDifferenceSpace}
+# Backwards-compatibility alias for the pre-rewrite ColumnField, which was a
+# Field with DataLayouts.DataColumn values. Single-column fields are now
+# identified by their space (a FiniteDifferenceSpace, which may wrap a
+# ColumnGrid returned by `column(::ExtrudedFiniteDifferenceSpace, ...)`), so
+# the old ColumnField is the same set of fields as FiniteDifferenceField.
+const ColumnField = FiniteDifferenceField
 const FaceFiniteDifferenceField{V, S} =
     Field{V, S} where {V <: DataLayout, S <: Spaces.FaceFiniteDifferenceSpace}
 const CenterFiniteDifferenceField{V, S} = Field{