Add submodule with special classes for Arbor and Neuron/LFPy+pt3d

[espenhgn]

Is your feature request related to a problem? Please describe.
The 3D geometry of a neuron model can be represented by an arbitrary number of data points (typically coordinates/diameter x, y, z, d along cable segments), while the number of electric compartments (or CVs in Arbor terms) may be more or less.
In "old" LFPy we considered only the start- and endpoint of straight-line electric compartments, while one can distribute transmembrane currents along the chain of frusta making up each compartment.
This functionality is exemplified e.g., in https://github.com/LFPy/LFPykit/blob/master/examples/Example_LFPy_pt3d.ipynb

Describe the solution you'd like
We could package inherited CellGeometry classes (for Arbor, Neuron, LFPy+pt3d) which track which frusta belong to each compartment, as well as slightly modified get_transformation_matrix methods for the different classes in lfpykit.models (see above notebook).

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