docs: public static void diag( DMatrix6x6 input , DMatrix6 out ) {
          out.a1 = input.a11;
          out.a2 = input.a22;
          out.a3 = input.a33;
          out.a4 = input.a44;
          out.a5 = input.a55;
          out.a6 = input.a66;
      }