ENH: clean the ML factories when application is deleted

Modules/Applications/AppClassification/app/otbImageClassifier.cxx

@@ -61,6 +61,13 @@ public:
   typedef otb::MachineLearningModelFactory<ValueType, LabelType>                               MachineLearningModelFactoryType;
   typedef ClassificationFilterType::ConfidenceImageType                                        ConfidenceImageType;
 
+protected:
+
+  ~ImageClassifier() ITK_OVERRIDE
+    {
+    MachineLearningModelFactoryType::CleanFactories();
+    }
+
 private:
   void DoInit() ITK_OVERRIDE
   {

Modules/Applications/AppClassification/include/otbLearningApplicationBase.h

@@ -135,6 +135,8 @@ public:
 protected:
   LearningApplicationBase();
 
+  ~LearningApplicationBase() ITK_OVERRIDE;
+
   /** Generic method to train and save the machine learning model. This method
    * uses specific train methods depending on the chosen model.*/
   void Train(typename ListSampleType::Pointer trainingListSample,

Modules/Applications/AppClassification/include/otbLearningApplicationBase.txx

@@ -30,7 +30,14 @@ template <class TInputValue, class TOutputValue>
 LearningApplicationBase<TInputValue,TOutputValue>
 ::LearningApplicationBase() : m_RegressionFlag(false)
 {
-} 
+}
+
+template <class TInputValue, class TOutputValue>
+LearningApplicationBase<TInputValue,TOutputValue>
+::~LearningApplicationBase()
+{
+  ModelFactoryType::CleanFactories();
+}
 
 template <class TInputValue, class TOutputValue>
 void