added the cpp version of mo_lib_laplace and compiled the code

added the cpp version of mo_lib_laplace and compiled the code.

src/horizontal/mo_lib_laplace.cpp

@@ -253,7 +253,11 @@ void nabla2_scalar_avg_lib(const T* psi_c, const int* cell_neighbor_idx, const i
                                         });
                  }
             }//if
-}//switch
+            break; 
+
+        default: 
+            std::cerr << "Unknown value for cell_type: " << cell_type << '\n';
+    }//switch
 }//void