Revert "Remove DOF multiplicity parloop"

This reverts commit 436bc44.

Author: pbrubeck

firedrake/interpolation.py

@@ -1047,12 +1047,16 @@ def _interpolator(tensor, expr, subset, access, bcs=None):
     if needs_weight:
         # Compute the reciprocal of the DOF multiplicity
         W = dual_arg.function_space()
-        m_ = get_interp_node_map(source_mesh, target_mesh, W)
-        m_indices = W.dof_dset.lgmap.apply(m_.values)
+        wsize = W.finat_element.space_dimension() * W.block_size
+        kernel_code = f"""
+        void multiplicity(PetscScalar *restrict w) {{
+            for (PetscInt i=0; i<{wsize}; i++) w[i] += 1;
+        }}"""
+        kernel = op2.Kernel(kernel_code, "multiplicity", requires_zeroed_output_arguments=False)
         weight = firedrake.Function(W)
+        m_ = get_interp_node_map(source_mesh, target_mesh, W)
+        op2.par_loop(kernel, cell_set, weight.dat(op2.INC, m_))
         with weight.dat.vec as w:
-            w.setValues(m_indices, numpy.ones(m_indices.shape), PETSc.InsertMode.ADD)
-            w.assemble()
             w.reciprocal()
 
         # Create a buffer for the weighted Cofunction and a callable to apply the weight