Improved incremental layout

We have two potential improvements for incremental layout:

[ ] 1. When tiled nodes are moved, their locations are used to determine the initial position of the tiled nodes as a whole but not individually. In below example I move a node to the previous row but it's always tiled the time when incremental layout is applied.
[x] 2. Degree 1 nodes (or in general nodes that are reduced as part of trees) are not laid out incrementally. See how they change places with succeeding incremental layouts.

iVis-at-Bilkent / cytoscape.js-fcose