message: In class BlockNonlinearForm, add methods to access the block
  offsets constructed by the class.
diff: @@ -127,6 +127,9 @@ public:
  /// After a call to SetSpaces(), the essential b.c. must be set again.
  void SetSpaces(Array<FiniteElementSpace *> &f);
  + const Array<int> &GetBlockOffsets() const { return block_offsets; }
  + const Array<int> &GetBlockTrueOffsets() const { return block_trueOffsets; }
  +
  /// Adds new Domain Integrator.
  void AddDomainIntegrator(BlockNonlinearFormIntegrator *nlfi)
  { dnfi.Append(nlfi); }
  