Added a function to test the interaction of two neighbouring cells. The test...

Added a function to test the interaction of two neighbouring cells. The test is called by passing the option -c N to the executable where N is the number of
particles to insert in each of the cells. 
The unsorted and sorted interactions are then computed and the accelerations are dumped in a file. The python script 'plot_sorted.py' plots the accelerations
to verify the accuracy.