Implementation of the 0D grid to test collisional processes.
An OMP_LOCK was added to the nodes to properly write perform the
scattering (it is weird that multiple threads work in the same node at
the same time, but in 0D happens everytime).
Added a new case to test the 0D geometry.
User Manual updated with the new options.
easy to use a file from a previous run without processing it into a
plain text file.
Although the previous method presented some updates for 1D small cases,
this is quite easy to do with any type of simulations and, in the
future, with different mesh formats.
Documentation updated properly.
3D Cartesian geometry also tested.
Documentation updated properly.
Added weighting probability in the injection of particles.
Files and types with 'Cyl' have been changed to '2DCyl' to better
differentiate between the two types of 2D geometry.
Solvers for charged and neutral particles in 2D Cartesian space.
Added solveds for 1D neutral particles (this branch is not the place to
do it, but it was a minor change).
User Manual updated with the new accepted options.
