In each iteration, number of collisions are calculate as a REAL variable
(collFrac) and stored in each cell. The number of collisions is
calculated as FLOOR(collFrac) and, if it is >1 collisions are computed
as usual. Per each collision calculated, 1.0 is removed from collFrac
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.
