Added a new file to contain the variables used to describe the grid
--> grid-variables.sh
Copied the pythia8.C macro from the ROOT tutorial directory
and checked that it works ok with the standalone setup

The build_packages.sh script now builds in sequence ;
  * HepMC
  * Pythia8
  * ROOT
into a build directory (using --prefix)
The directory is tarred up and is attempted to be registered on the LFC
If there is no LFC_HOST set, the script will attempt to store it directly
in the /tmp directory of one of the found storage elements

Note: there is a lot of cleanup to be done, but the code should work.