getneighborhood_fast.Rdan efficient function to find the neighborhood based on the matrix of position and a pre-defined cutoff
getneighborhood_fast(x, radius)is a n-by-2 matrix of position.
is a threashold of Euclidean distance to decide whether a spot is an neighborhood of another spot. For example, if the Euclidean distance between spot A and B is less than cutoff, then A is taken as the neighbourhood of B.
A sparse matrix containing the neighbourhood