The geometry and push structure has been reworked to allow eassy adding
new pushers.
Documentation not updated yet.
Baseline for merging Cartesian pushers into one.
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.
