This function computes, for each point X[i] in X
and for each radius r,
the area of that part of the disc of radius r centred at the
location X[i] that is not covered by any of the
other discs of radius r centred at the points X[j]
for j not equal to i.
This area is important in some calculations related to
the area-interaction model AreaInter.
The result is a matrix, with one row for each point in X
and one column for each entry of r.