ENH: Add NumberOfWorkUnits to ElastixMain, used by CreateComponent

[N-Dekker]

With this commit, the GenerateData() member function of both TransformixFilter and ElastixRegistrationMethod pass their NumberOfWorkUnits to their corresponding ElastixMain/TransformixMain object, which passed this NumberOfWorkUnits to each component that is an itk::ProcessObject.

Related to issue InsightSoftwareConsortium/ITKElastix#163 "No option to set number of threads for Transformix", by Sebastian (@Svdvoort)

[N-Dekker]

For the record, five out of the ten elastix component base classes (at https://github.com/SuperElastix/elastix/tree/3e89c0cf9c90998b336fbed22c92d9f42bcf2959/Core/ComponentBaseClasses/) have an ITKBaseType that is derived (directly or indirectly) from itk::ProcessObject:

elastix component base class	ITKBaseType	inheritance
FixedImagePyramidBase	itk::MultiResolutionPyramidImageFilter<>	ImageToImageFilter => ImageSource => ProcessObject
MovingImagePyramidBase	itk::MultiResolutionPyramidImageFilter<>	ImageToImageFilter => ImageSource => ProcessObject
ImageSamplerBase	itk::ImageSamplerBase<>	ImageToVectorContainerFilter => VectorContainerSource => ProcessObject
RegistrationBase	itk::MultiResolutionImageRegistrationMethod2<>	ProcessObject
ResamplerBase	itk::ResampleImageFilter<>	ImageToImageFilter => ImageSource => ProcessObject

The other five are not derived from itk::ProcessObject:

elastix component base class	ITKBaseType	inheritance
InterpolatorBase	itk::InterpolateImageFunction<>	ImageFunction => FunctionBase => Object
ResampleInterpolatorBase	itk::InterpolateImageFunction<>	ImageFunction => FunctionBase => Object
MetricBase	itk::SingleValuedCostFunction	CostFunction => Object
OptimizerBase	itk::Optimizer	Object
TransformBase	AdvancedCombinationTransform	AdvancedTransform => Transform => TransformBaseTemplate => Object