GroupGraph: add field name to validate function for reading parameters from options dialog.
GroupGraph: option to enable snapping node positions to grid upon mouse up event.
GroupGraph: snap to grid must work for multiple nodes when selecting more than one,
and must ignore selected nodes when the dragged node is not selected
(which is has always been possible, surprisingly, in cytoscapejs).
GroupGraph: fix error in measuring risk.

Also, compute risk considering only inputs (except for computing the
flow centrality, which uses all actual output synapses).
GroupGraph: simplify function _regroup.
GroupGraph: fix error, wasn't showing edges between groups.