Add RS-coarsening tests

Author: gojakuch

core/multigrid/rs.cpp

@@ -115,7 +115,10 @@ void Rs<ValueType, IndexType>::generate()
     exec->run(rs::make_fill_seq_array(restrict_op->get_row_ptrs(),
                                       coarse_dim_size + 1));
 
-    auto prolong_op = gko::as<csr_type>(share(restrict_op->transpose()));
+    auto prolong_op = gko::as<csr_type>(
+        share(restrict_op->transpose()));  // is that correct? or should we use
+                                           // some interpolation to compute the
+                                           // prolongation operator?
 
     //
     auto coarse_matrix = share(

reference/multigrid/rs_kernels.cpp

@@ -52,6 +52,7 @@ void compute_soc_row_ptrs(std::shared_ptr<const ReferenceExecutor> exec,
         for (IndexType jj = a_row_ptrs[i]; jj < a_row_ptrs[i + 1]; ++jj) {
             const auto j = a_col_idxs[jj];
             if (j != i) {
+                // assuming an M-matrix
                 max_offdiag = std::max(max_offdiag, -real(a_vals[jj]));
             }
         }