Extension build error for Slicer 5.11 Preview

[jamesobutler]

https://slicer.cdash.org/viewBuildError.php?buildid=4071041

error C1083: Cannot open include file: 'SimpleITK.h': No such file or directory

The SimpleITK C++ project is no longer being built by Slicer core as its primary usage to generate the corresponding python whl was replaced by installing SimpleITK from PyPI. Slicer/Slicer@f87c977

SlicerNetstim appears to have been the only Slicer extension utilizing SimpleITK from its C++ interface. I would suggest utilizing only the ITK C++ interface so that SlicerNetstim can build successfully again for the Slicer 5.11 preview.

Relevant code:

SlicerNetstim/FiducialRegistrationVariableRBF/FiducialRegistrationVariableRBF.cxx

Lines 1 to 10 in 4e3d55a

	// The logic of this computation is taken from plastimatch and modified to use variable RBF Radius.
	// Instead of using plastimatch classes, here ITK and SimpleITK abstractions are used.
	// https://gitlab.com/plastimatch/plastimatch
	#include "FiducialRegistrationVariableRBFCLP.h"
	// ITK includes
	#include <itkImage.h>
	#include "SimpleITK.h"

[ningfei]

Thanks for the heads up! Would you be open to submitting a PR?