Calculates the Mean Squared Displacement for a trajectory
     
    
    computeMSD(sx, sy, until = 4)
 
    
    Arguments
- sx
- x axis positions along the trajectory 
- sy
- y axis positions along the trajectory 
- until
- how many points should be included in the Mean Squared Displacement curve 
 
    
    Value
    A numeric vector containing the values of the MSD