Add clustering/classification capacity

[Edouard-Legoupil]

It should be possible to also define within the xlsform -- or indicator calculation sheet - the variables that could be used together to build some clustering

The output would be part of the final Rmd and could allow to compare different clustering algorithm (hierarchical clustering, k-means clustering, latent-class modeling...)

[Edouard-Legoupil]

https://github.com/unhcr/koboloadeR/blob/master/R/kobo_cluster_report.R