make_range() function extend param

[tanghaibao]

http://github.com/tanghaibao/quota-alignment/blob/master/cluster_utils.py#L139

Although quite uncommon - when extend is set to large a negative value - non-overlapping segments, after correcting the boundaries, can become overlapping. This is not intended behavior.

For example, say two segments (10, 20), (30, 40). and set the extend to be -100 (tolerating distance Nmax 100), becomes (-80, 10) and (-60, 30), thus overlapping.

since we correct all the boundaries before other calculations. not sure how to fix this.