STYLE: ITK_FUTURE_LEGACY_REMOVE/deprecate MultiThreaderType aliases

Asked users to use `itk::MultiThreaderBase` directly.

Author: N-Dekker

Modules/Core/Common/include/itkProcessObject.h

@@ -164,8 +164,12 @@ class ITKCommon_EXPORT ProcessObject : public Object
   /** STL array of data object names */
   using NameArray = std::vector<DataObjectIdentifierType>;
 
-  /** Type of general multi-threader interface */
-  using MultiThreaderType = MultiThreaderBase;
+#ifndef ITK_FUTURE_LEGACY_REMOVE
+  /** Type of general multi-threader interface
+   * \deprecated Please use `itk::MultiThreaderBase` directly.
+   */
+  using MultiThreaderType [[deprecated("Please use `itk::MultiThreaderBase` directly.")]] = MultiThreaderBase;
+#endif
 
   /** \brief Return an array with the names of the inputs defined.
    *

Modules/Registration/Common/include/itkImageToImageMetric.h

@@ -349,7 +349,11 @@ class ITK_TEMPLATE_EXPORT ImageToImageMetric : public SingleValuedCostFunction
   itkGetConstReferenceMacro(UseCachingOfBSplineWeights, bool);
   itkBooleanMacro(UseCachingOfBSplineWeights);
   /** @ITKEndGrouping */
-  using MultiThreaderType = MultiThreaderBase;
+
+#ifndef ITK_FUTURE_LEGACY_REMOVE
+  using MultiThreaderType [[deprecated("Please use `itk::MultiThreaderBase` directly.")]] = MultiThreaderBase;
+#endif
+
   /** Get the Threader. */
   /** @ITKStartGrouping */
   itkGetModifiableObjectMacro(Threader, MultiThreaderBase);