BUG: SetIdentity must retain FixedParameters

When setting a transform to represent an Identity
mapping, it is crucial that the stationary component
of the transform is preserved.

```python
tfm.SetCenter( [1,2,3] );
tfm.SetIdentity();
out=tfm.GetCenter();
assert( out == [1,2,3] );
```

===============

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/include/itkMatrixOffsetTransformBase.hxx

@@ -95,6 +95,8 @@ template <typename TParametersValueType, unsigned int VInputDimension, unsigned
 void
 MatrixOffsetTransformBase<TParametersValueType, VInputDimension, VOutputDimension>::SetIdentity()
 {
+  // Fixed parameters must be preserved when setting the transform to identity
+  const auto tmp_fixed_parameters = this->GetFixedParameters();
   m_Matrix.SetIdentity();
   m_MatrixMTime.Modified();
   m_Offset.Fill(OutputVectorValueType{});
@@ -104,6 +106,7 @@ MatrixOffsetTransformBase<TParametersValueType, VInputDimension, VOutputDimensio
   m_InverseMatrix.SetIdentity();
   m_InverseMatrixMTime = m_MatrixMTime;
   this->Modified();
+  this->SetFixedParameters(tmp_fixed_parameters);
 }
 
 

Modules/Core/Transform/test/itkEuler3DTransformGTest.cxx

@@ -33,3 +33,20 @@ TEST(Euler3DTransform, SetFixedParametersThrowsWhenSizeIsLessThanInputSpaceDimen
     ASSERT_THROW(transform->SetFixedParameters(fixedParameters), itk::ExceptionObject);
   }
 }
+
+TEST(Euler3DTransform, TestSetGetCenterAfterSetIdentity)
+{
+  using EulerTransformType = itk::Euler3DTransform<>;
+
+  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);
+  EXPECT_EQ(eulerTransformWithCenter->GetCenter(), centerOfRotation);
+  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.
+  EXPECT_EQ(eulerTransformWithCenter->GetCenter(), centerOfRotation);
+}