message: added const and non-const versions.
diff: @@ -730,7 +730,9 @@ public:
  double *GetData(int k) { return tdata+k*Mk.Height()*Mk.Width(); }
  - double *Data() const { return tdata; }
  + double *Data() { return tdata; }
  +
  + const double *Data() const { return tdata; }
  /** Matrix-vector product from unassembled element matrices, assuming both
  'x' and 'y' use the same elem_dof table. */
  