ENH: Add test demonstrating change in fixed paramers

When applying SetIdentity(), the fixed parameters should not be modified.
The fixed paramers represent the stationary component of the transform
and setting the transform back to an identity should not modify the
stationary components.

Modules/Core/Transform/test/itkEuler3DTransformTest.cxx

@@ -34,10 +34,34 @@ itkEuler3DTransformTest(int, char *[])
   using EulerTransformType = itk::Euler3DTransform<double>;
   auto eulerTransform = EulerTransformType::New();
 
+  // Testing preservation of the center of rotation
+  auto                        eulerTransformWithCenter = EulerTransformType::New();
+  const itk::Point<double, 3> centerOfRotation({ 200, 400, 300 });
+  eulerTransformWithCenter->SetCenter(centerOfRotation);
+  std::cout << "Testing Set/GetCenter(): ";
+  if (eulerTransformWithCenter->GetCenter() != centerOfRotation)
+  {
+    std::cerr << "Error in Set/GetCenter() " << std::endl;
+    return EXIT_FAILURE;
+  }
+  std::cout << "[ PASSED ]" << std::endl;
+  std::cout << "Testing GetCenter() after SetIdentity(): ";
+  eulerTransformWithCenter->SetIdentity();
+  // The center of rotation should be preserved when the transform is set to identity.
+  // Reseting a transform to identity should not affect the FixedParameters.
+  std::cout << "Testing Set/GetCenter(): " << std::endl;
+  if (eulerTransformWithCenter->GetCenter() != centerOfRotation)
+  {
+    std::cerr << "Error in GetCenter() after calling SetIdentity() " << std::endl;
+    return EXIT_FAILURE;
+  }
+  std::cout << "[ PASSED ]" << std::endl;
+
   // Testing Identity
   std::cout << "Testing identity transform: ";
   eulerTransform->SetIdentity();
 
+
   EulerTransformType::OffsetType offset = eulerTransform->GetOffset();
   if (offset[0] != 0.0 || offset[1] != 0.0 || offset[2] != 0.0)
   {