message: Add HasSpMatElim to check if mat_e is null. Useful check before trying to use SpMatElim, which will fail if mat_e is null
diff: @@ -337,6 +337,12 @@ public:
  return *mat_e;
  }
  + // Returns true if the sparse matrix of eliminated b.c.s is not null, false otherwise.
  + bool HasSpMatElim()
  + {
  + return mat_e != nullptr;
  + }
  +
  /// Adds new Domain Integrator. Assumes ownership of @a bfi.
  void AddDomainIntegrator(BilinearFormIntegrator *bfi);
  /// Adds new Domain Integrator restricted to certain elements specified by
  