ENH: Add new otbTrainImagesClustering CompositeApplication.

Use inheritance with otbTrainImagesBase to create unsupervised
composite application.

Modules/Applications/AppClassification/app/otbTrainImagesClustering.cxx

+#include "otbTrainImagesBase.h"
+
+namespace otb
+{
+namespace Wrapper
+{
+
+class TrainImagesClustering : public TrainImagesBase<false>
+{
+public:
+  typedef TrainImagesClustering         Self;
+  typedef TrainImagesBase<false>        Superclass;
+  typedef itk::SmartPointer<Self>       Pointer;
+  typedef itk::SmartPointer<const Self> ConstPointer;
+  itkNewMacro( Self )
+  itkTypeMacro( Self, Superclass )
+};
+
+}
+}
+
+OTB_APPLICATION_EXPORT( otb::Wrapper::TrainImagesClustering )
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