Dendritic spine: Some class members marked private on C++ side can be access on Python side

[jacquemi-bbp]

This issue concerns a closed PR: #321

There should be no Mitochondria and no Endoplasmic Reticulum object attached to Dendritic Spine. But they exist because the DendriticSpine class derived from the Morphology class.
So they are marked private on c++ implementation so one cannot access them.
The problem is that in Python one can access these members.

[eleftherioszisis]

Given that in biology smooth endoplasmic reticulum and mitochondria are present in dendritic spines, it makes sense that these objects are accessible from the spine morphology, doesn't it?