add how we construct DL_POLY file

Doc/Src_tex/ScalFmm-PeriodicModel/periodicmodel.tex

@@ -421,7 +421,7 @@ Force &$C_{force}$& -138935.4835 &  $10\,J  mol^{-1}\, \AA^{-1}$\\
 The force unit is the internal DL\_Poly unit.
 
 
-The first test consists in a small crystal $4\times 4\times 4$ of NaCl. It is composed of 128 atoms The second tests is a larger crystal  $10\times 10\times 10$ of NaCl and have X atoms.
+The first test consists in a small crystal $4\times 4\times 4$ of NaCl. It is composed of 128 atoms The second tests is a larger crystal  $10\times 10\times 10$ of NaCl and have 2000 atoms. The positions and the forces are stored in the \texttt{REVCON} file and the energy in the  \texttt{STATIS} file.
 
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 \section{Conclusion}