message: Add DenseTensor::Swap
diff: @@ -859,6 +859,12 @@ public:
  double *HostReadWrite()
  { return mfem::ReadWrite(tdata, Mk.Height()*Mk.Width()*nk, false); }
  + void Swap(DenseTensor &t)
  + {
  + mfem::Swap(tdata, t.tdata);
  + mfem::Swap(nk, t.nk);
  + }
  +
  ~DenseTensor() { tdata.Delete(); }
  };
  