Based on previous similutation work, the function suggests an optimal
value for the N parameter based on national prevalence, the total number
of observations and the number of clusters. See Larmarange et al. 2011
for more details.
Larmarange Joseph, Vallo Roselyne, Yaro Seydou, Msellati Philippe and Meda Nicolas (2011)
"Methods for mapping regional trends of HIV prevalence from Demographic and Health Surveys (DHS)",
Cybergeo: European Journal of Geography, no 558, http://cybergeo.revues.org/24606,
DOI: 10.4000/cybergeo.24606.