Computes the distance 2-point correlation function of a finite 2D lattice. The functions is:
[ coorfun r rw] = twopointcorr(x,y,dr,blksize,verbose)

Where x is the list of x coordinates of lattice points. y is the list of y coordinates of the lattice points,and dr is the binning length for the correlation function. blksize is the number of points processed at a time. verbose is a boolean on whether to display current progress.

The outputs are coorfun, r, and rw. Coorfun is a vector of normalized correlation function values for distances recorded in the r vector. rw is the weight (number of points) contributing to corrfun calculation at a particular r. Low rw means that corrfun at that r value is unreliable.

This function has been fully vectorized to improve performance.