Restructure the folders, simplify the website, prepare for GUI

[simongravelle]

Done:

• restructure the folders from level1, level2, level3 to tutorial1, tutorial2, etc
• remove the "justify" css
• divided each tutorial into "introduction.rst", "tutorial.rst", and "exercise.rst " files
• added @jrgissing and @akohlmey as authors
• removed the VMD, MDA, and bash short tutorial (they were move to dedicated github repository)

To be done:

• convert .lammps file to .lmp
• adapt tutorial to GUI
• copy past some of the content from the article into the website

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...