Vectorize calculation of axial currents

[espenhgn]

The calculation of axial currents (Algorithm 1 in https://doi.org/10.3389/fninf.2018.00092) is linear, that is, the I = A*V_1 + B*V_2 + C*... where the coefficients A, B, C, ... are computed from the corresponding compartment axial resistivity. The calculation of axial currents from membrane voltages can be reformulated as I = MV where V is a matrix with all voltages, M a sparse matrix with the corresponding coefficients that can be computed once. The calculation of axial currents can then be computed very fast without explicit for loops.

@solveignaess, does this sound feasible?

[solveignaess]

Yes, I think this sounds feasible. However, finding the axial resistance is not so straightforward (needs to be set specifically for first or last segment in a section, and for segments attached to more than one segment), and I think some looping might be necessary in order to do this. But I don't think we need to loop through all segments of each section, so it seems like we can save time here!

[espenhgn]

Constructing the sparse matrix M will indeed involve nested for-loops I agree, but it only needs to be computed once, and wouldn't be slow relative to the time it takes to solve for membrane voltages and transmembrane currents in a typical simulation.

[solveignaess]

Yes, of course, now I see! We only need one M independent of the number of timesteps.=) I really think we should do this.

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