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.
Unification of boundary conditions into one file.
Some changes to input file for reference cases. This should have been
done in another branch but I wanto to commit to save progress and I
don't want to deal with tswitching branches right now, I'm very busy
watching Futurama.
