x/r position to avoid huge fields in Radial case when r -> 0 due to
charge accumulation.
Added a Gnuplot script to plot both potential profiles extracted from
Gmsh.
A boundary condition for each species must be indicated in the case
file.
This opens the door to use boundary conditions with different parameters
(for example, a wall temperature, coefficients for reflection or
absorption...)
The examples included with the code have been updated accordently.
