Neuron demo file to simulate local field potentials

This Neuron demo file simulates the model of local field potentials
(LFP) with variable resistivity, as described in the following paper:
Bedard C, Kroger H and Destexhe A. Modeling extracellular field
potentials and the frequencyfiltering properties of extracellular
space. Biophysical Journal 86: 18291842, 2004.
This paper develops a model of LFP that includes the frequency
filtering properties of extracellular space. The paper considers
inhomogeneous conductivity and permittivity profiles, which results
from the multiple media (fluids, membranes, vessels, ...) composing
the extracellular space around neurons. Including nonconstant
profiles of conductivity enables the model to display frequency
filtering properties, ie slow events such as EPSPs/IPSPs are less
attenuated than fast events such as action potentials.
The model shown here is a reproduction of Figure 6 of the paper.
The source current is monopolar in this simple case and consists of
an EPSP/IPSP sequence followed by an action potential (see details
in the paper).
The method to calculate the extracellular LFP consists of the
following steps:
 read in the source current I from a data file
 compute the Fourier transform of the source current I
 compute the impedance Z assuming a nonhomogeneous conductivity
function (radial symetry)  this step requires a MOD file
with the ImpedanceFM mechanism
 compute the extracellular voltage by inverse Fourier transform
of the product Z(w) * I(w)
Note that this demo file is provided "as is", unfortunately I have
no time to help implementing, but feedbacks are always welcome if
it has been useful to you. If you use this for your research,
please cite the above paper.
Alain Destexhe,
CNRS, Gif sur Yvette, France
http://cns.iaf.cnrsgif.fr
Changelog

202205: Updated MOD files to contain valid C++ and be compatible
with the upcoming versions 8.2 and 9.0 of NEURON.
