[PHIEstimator] delete itk smart pointer the correct way

[dboun]

Let me preface this by saying that I'm not exactly sure about this, but here it goes.

1. Turns out the correct way to delete itk SmartPointer(s) is smartPointer = NULL; [ITK REFERNCE - look at "Coding Style">"DO NOT"]

2. mitk::Pointer(s) are typedef'd itk::SmartPointer(s)

3. [HERE IN PHIEstimator] there is a maskImageTmp->Delete(); which should probably be mitkImageTmp = NULL.

4. I think the "issue" is that if you call Delete() the smart pointer will be destroyed regardless of the number of references it holds, while the = NULL just decreases the reference counter by one and deletes it only if it reaches 0. It doesn't look like it matters here, but we should stick to the correct ways.