Prepare the LAMMPS tutorial website to the new LAMMPS stable release

[simongravelle]

To be done:

• there is no need for the custom css anymore, so the .rst files will be simplified
• convert .lammps file to .lmp
• adapt tutorial to GUI
• removed the VMD, MDA, and bash short tutorial --> they were move to dedicated github repository
• restructure the folders from level1, level2, level3 to tutorial1, tutorial2, etc

Open question:

• how much of the changes applied to the article should be incorporated to the website? this includes GUI, type label, general restructuring of the inputs...