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 r : float or onedimensional array of floats
p : float, 1<=p<=infinity


Returns :  result : integer or onedimensional array of integers
