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] );
```

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{});
@@ -103,7 +105,7 @@ MatrixOffsetTransformBase<TParametersValueType, VInputDimension, VOutputDimensio
   m_Singular = false;
   m_InverseMatrix.SetIdentity();
   m_InverseMatrixMTime = m_MatrixMTime;
-  this->Modified();
+  this->SetFixedParameters(tmp_fixed_parameters);
 }