missing PatientPosition::operator!=

[KrisThielemans]

This leads to quite a surprise in Python:

>>> exam_info.patient_position.this
<stir::PatientPosition::HFS>
>>> exam_info.patient_position == stir.PatientPosition(stir.PatientPosition.HFS) 
True
>>> exam_info.patient_position == stir.PatientPosition(stir.PatientPosition.FFS)
False
>>> exam_info.patient_position != stir.PatientPosition(stir.PatientPosition.HFS) 
True
>>> exam_info.patient_position != stir.PatientPosition(stir.PatientPosition.FFS) 
True

The 3rd one is "wrong". It is probably caused by Python's default __ne__ being called, which checks in terms of identity (not equality).