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.
Now collisions can have a different time step.
Added species name to output names as it was starting to get confusing
in Gmsh for multiple species.
Output filenames adapted to match any number of iterations.
