Add Coordinates::communicateDz() and call after constructing Coordinates

to prevent getUniform(coords->zlength()) failing because the field is not const.

Author: tomchapman

include/bout/coordinates.hxx

@@ -237,6 +237,8 @@ public:
 
   void communicateMetricTensor();
 
+  void communicateDz();
+
   ///< Coordinate system Jacobian, so volume of cell is J*dx*dy*dz
   FieldMetric& J() const;
 

include/bout/mesh.hxx

@@ -654,6 +654,7 @@ public:
                                                 force_interpolate_from_centre);
 
     inserted.first->second->communicateMetricTensor();
+    inserted.first->second->communicateDz();
 
     return inserted.first->second;
   }

src/mesh/coordinates.cxx

@@ -1540,4 +1540,8 @@ void Coordinates::setMetricTensor(
 void Coordinates::communicateMetricTensor() {
   contravariantMetricTensor.communicate();
   covariantMetricTensor.communicate();
+}
+
+void Coordinates::communicateDz() {
+  localmesh->communicate(dz_);
 }