Count how many nearby pairs can be formed.
Count the number of pairs (x1,x2) can be formed, with x1 drawn from self and x2 drawn from other, and where distance(x1, x2, p) <= r. This is the “twopoint correlation” described in Gray and Moore 2000, “Nbody problems in statistical learning”, and the code here is based on their algorithm.
Parameters :  other : KDTree instance
r : float or onedimensional array of floats
p : float, 1<=p<=infinity


Returns :  result : int or 1D array of ints
