Acknowledgements
The authors sincerely thank “Université Catholique de Louvain(UCL)” for the grant provided to Mr. LONTIO FOMEKONG Roussinthrough the scholarship program “cooperation au développement”to enable him travel to Belgium where the main part of theresearch in this paper was carried out. The authors also acknowl-edge the financial support of WBGREEN programs (Walloon Regionof Belgium).