ENH: Add getter methods for itk::KdTreeGenerator publicly set ivars

Add getter methods for `itk::KdTreeGenerator` publicly set source sample
and bucket size ivars.

Test the methods.

Modules/Numerics/Statistics/include/itkKdTreeGenerator.h

@@ -110,14 +110,16 @@ class ITK_TEMPLATE_EXPORT KdTreeGenerator : public Object
   /** Typedef for the smart pointer to the Subsample */
   using SubsamplePointer = typename SubsampleType::Pointer;
 
-  /** Sets the input sample that provides the measurement vectors. */
+  /** Set/Get the input sample that provides the measurement vectors. */
   void
   SetSample(TSample * sample);
+  itkGetConstMacro(SourceSample, TSample *);
 
   /** Sets the number of measurement vectors that can be stored in a
    * terminal node. */
   void
   SetBucketSize(unsigned int size);
+  itkGetConstMacro(BucketSize, unsigned int);
 
   /** Returns the pointer to the generated k-d tree. */
   OutputPointer

Modules/Numerics/Statistics/test/itkKdTreeGeneratorTest.cxx

@@ -49,7 +49,12 @@ itkKdTreeGeneratorTest(int, char *[])
 
 
     treeGenerator->SetSample(sample);
-    treeGenerator->SetBucketSize(16);
+    ITK_TEST_SET_GET_VALUE(sample, treeGenerator->GetSourceSample());
+
+    unsigned int bucketSize = 16;
+    treeGenerator->SetBucketSize(bucketSize);
+    ITK_TEST_SET_GET_VALUE(bucketSize, treeGenerator->GetBucketSize());
+
     treeGenerator->Update();
 
     using TreeType = TreeGeneratorType::KdTreeType;