Change the way QuadTree.getLeafUnder() locate the leaf.
* Change: In cumulus/plugins.Block.getLeafUnder(): formerly, we where using the cut-lines (x/y) to locate which leaf a point is under. But in case of incomplete tree, it is difficult to manage. Now we chosse the leaf according to the distance to the center of the leaf area. Choose the shorter, of course. This solve the H-Tree DFF bad assignment around the PLL (top right corner) in LS180.