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.
