Refactor dependencies

[GregorDeCillia]

• dplyr: Only used once in the package (see below) so we could really remove the dependency altogether.
• matrixStats: only used for matrixStats::rowProds. Should be removed completely
• ggplot2: The plotting features are not very central for the package so we could move the dependency from Imports to Suggests
• laeken: Used for example data (eusilc), gini() and weightedMedian(). Probably we should keep this dependency as-is. Otherwise, we would have to host our own copy of the laeken::eusilc data
• Rcpp: Necessary
• data.table: Necessary

surveysd/R/demo.eusilc.R

Line 74 in 8c64f39

eusilc[, ecoStat := dplyr::recode(

[GregorDeCillia]

It turns out, changing ggplot2 to an optional dependency is not as easy as expected because of this

surveysd/R/helpers.R

Line 4 in 735980f

#' @import ggplot2

Namespace imports are not allowed for optional dependencies which means that quite some changes in R/plot.R would be necessary to implement this.