basic global thresholding

this function is to estimate a threshold using the basic global threshholding algorithm

The input is a vector x, the output is an estimated threshold T, which groups x into two clusters using the basic global thresholding
procedures:
1)Randomly select an initial estimate threshold T.
2)Segment the signal using T, which will yield two groups, G1 consisting of all points with values<=T and G2 consisting of points with value>T.
3)Compute the average distance between points of G1 and T, and points of G2 and T.
4)Compute a new threshold value T=(M1+M2)/2
5)Repeat steps 2 through 4 until the change of T is smaller enough.