Initial commit.

- Scripts for generating png images of different graphics. The extension
  used for gnuplot script is 'gp'.
- The png images obtained from the scripts are also added for those who
  do not want to install graphviz.
- The data files from which the graphics are generated (with extension
  .dat)
- A bash script 'generer-graphiques' for calling gnuplot on every
  script.