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.
Allow arbitrary zoom levels via text input

This allows zoom levels such as "0.5" to be typed into the zoom text
input while preserving the discrete level snapping behavior of the zoom
slider. The slider handle reflects manually input zoom levels. Manually
input zoom levels are clamped within the extrema of zoom levels
configured for the slider. Closes acardona/CATMAID#686.
Slider: add major and minor steps for finer zoom

Slider now optionally takes two sets of steps: major and minor. Sliders
normally behave by snapping to minor steps during scrolling and bound
key action. Modifying any of these actions with shift will snap to major
steps. By default major and minor steps are identical for compatibility.

The previous integer zoom levels are now major steps, and minor steps
are in tenths. Major steps every 10 slices were added for z navigation.
All other sliders are unchanged.