Nearest neighboring particle search using all particles search

To get the list of nearest neighbors around a given particle

In mesh less methods like SPH (smoothed particle hydrodynamics) it is needed to get the list of neighboring particles lying with in a certain length of a given point.These particles are referred as nearest neighboring particles (NNP)of the concerned particle. The present code finds the list of NNP for all the particles. The algorithm used is all pairs search. Distance between a concerned point and all the points is calculated and points are sorted out depending on the given length. This algorithm is time consuming, it is of order O(N*N) where N is total number of particles. But it helps in understanding mathematics behind and is a good start to know the concepts.