ENH: Add helper value for number of inputs

Modules/Core/Functor/include/otbFunctorImageFilter.h

@@ -210,7 +210,7 @@ public:
   using InputHasNeighborhood = typename FunctorFilterSuperclassHelper<TFunction>::InputHasNeighborhood;
   using InputTypesTupleType = typename Superclass::InputTypesTupleType;
   template<size_t I> using InputImageType = typename Superclass::template InputImageType<I>;
-
+  using Superclass::NumberOfInputs;
   
   /** Run-time type information (and related methods). */
   itkTypeMacro(FunctorImageFilter, ImageToImageFilter);

Modules/Core/Functor/include/otbVariadicInputsImageFilter.h

@@ -37,6 +37,8 @@ public:
   using InputTypesTupleType = std::tuple<TInputs...>;
   
   template <size_t I> using InputImageType = typename std::tuple_element<I,InputTypesTupleType>::type;
+  static constexpr size_t NumberOfInputs = std::tuple_size<InputTypesTupleType>::value; 
+
   
   itkNewMacro(Self);