Create average population in 3D -updated-

@tarelli @stephenlenzi this is to track our current (lack of) understanding of Martyn's request to average neuronal centroids in 3D and show them as new centroids? Let's start from noting down what is our understanding of his request

Start by reading the issue discussion, since the request currently records a lack of understanding rather than a defined change. Clarify how neuronal centroids should be averaged in 3D and how the resulting centroids should be displayed, then identify the relevant project entry points. Done means the behavior and expected output are agreed and testable.

Written by the indexing model from the issue text.