Collect pt3d info per segment, not per section

[espenhgn]

Is your feature request related to a problem? Please describe.
Now, the pt3d info is collected per NEURON Section object (in LFPy.Cell._collect_pt3d() as we've mostly used this info for plotting, not predictions.

It would be better if we could approximately map the fine geometrical detail which may be present in the reconstructed morphology files to the corresponding segment reference, so that it can be used a bit more readily for FM predictions. This would imply that we can consider each segment as a series of conical frusta (as do Arbor) with constant transmembrane current per area.

[github-actions]

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