create geographic coarsening functions

[CarlijnB]

Functions for network privacy - geographic coarsening (pseudocode from @magicicada w input from @mdenwood) :

get_convex_hull(locations, number, central):
generate convex hull that contains central and number other points (closest number locations to central?)

choose_within_shape(hull):
chooses uniformly at random a point within the hull (optionally using spatial / land usage data provided via HexScape / corine as a mask for farmland or eurostat for country borders)

[CarlijnB]

might be needed: helper function(s) to deal with country-specific grid systems