Count number of point data nearby a regular grid point

[boriskaus]

this function is useful to create a 3D map of earthquake events, which can be used to generate model setups.

example:

julia> Grid_cart = CartData(XYZGrid(-20:20,-20:.1:20,-30:30))
CartData 
    size    : (41, 401, 61)
    x       ϵ [ -20.0 : 20.0]
    y       ϵ [ -20.0 : 20.0]
    z       ϵ [ -30.0 : 30.0]
    fields  : (:Z,)

#Create some random point data
julia> pt = rand(10_000,3) .- 0.5;
julia> pt[:,1] .*= 20;
julia> pt[:,2] .*= 20;
julia> pt[:,3] .*= 30;

julia> Grid_Count = PointData2NearestGrid(pt[:,1],pt[:,2],pt[:,3], Grid_cart; radius_factor=2)
CartData 
    size    : (41, 401, 61)
    x       ϵ [ -20.0 : 20.0]
    y       ϵ [ -20.0 : 20.0]
    z       ϵ [ -30.0 : 30.0]
    fields  : (:Z, :Count)