DOC: Fix ingroup tags

Modules/Core/Functor/include/otbVariadicAddFunctor.h

@@ -31,6 +31,7 @@ namespace Functor
 /**
  * \class VariadicAdd
  * \brief This functor adds any number of compile time scalar inputs 
+ * \ingroup OTBFunctor
  */
 template <typename TOut, typename ...TIns> struct VariadicAdd
 {

Modules/Core/Functor/include/otbVariadicConcatenateFunctor.h

@@ -80,6 +80,7 @@ template <typename v1, typename v2, typename ...vn> void concatenateVectors(v1 &
  * \class VariadicConcatenate
  * \brief This functor concatenates any number of input of scalar type
  * or VariableLengthVector.
+ * \ingroup OTBFunctor
  */
 template<typename TOut, typename ...TIns> struct VariadicConcatenate
 {