New tutorial for the density_cylinder module

** 1) Added tutorial for density cylinder **

I added a small tutorial for the density cylinder module.

** 2) Cleaned density_planar' docstrings **

I cleaned the density_planar' docstrings, ie I shifted all text by one 'tabulation' to the left (Before, the documentation was all messed up, and after some testing, it seems like its the only solution to end-up with a nice documentation).