centroid returns incorrect result when multiplogon consists of polygon in different clockwise

[xlqian]

Consider:
A Multi polygon consists of two polygon [p(0., 0.), p(2., 0.), p(2., 2.), p(0., 2.), p(0., 0.)] (clockwise) and [p(0., 0.), p(-2., 0.), p(-2., 2.), p(0., 2.), p(0., 0.)](anti-clockwise), whose centroid is (0,1).

Current computation gives an incorrect result (Nan, Nan).

[frewsxcv]

Fixed in #91