Touch up bunch; work on Poisson solver; add cython functions for grid computations.
Looks like working particle distribution in grid_functions.pyx.
Add compute_potential_fgreenm2m.pyx.
Remove green_m2m_old.pyx.
Implement density computation using histogram.
Extend beam module by base and new derived classes.
Added in setup.py the commands able to delete Cython files for PC
running Windows; setup.py can be now launched without problems also in
Windows with the compiler TDM-GCC (read comment)