Fix issue with ExternalFaces on CellSetSingleType

The ExternalFaces filter should work fine on a CellSetSingleType.
However, the filter was not operating on them because it a templated
method that is a placeholder for "this cell set type not supported" was
matching it. This simple change allows a CellSetSingleType to match the
template for a CellSetExplicit (which is the superclass).

vtkm/filter/ExternalFaces.hxx

@@ -48,8 +48,7 @@ public:
     *this->Valid = true;
   }
 
-  template<typename T>
-  void operator()(const T& ) const
+  void operator()(const vtkm::cont::CellSet& ) const
   {
     //don't support this cell type
     *this->Valid = false;