Crashes on graphs with disconnected components when using weak gravity

[govertb]

E.g. using ./graph_viewer gpu 500 0 wg 5 1 approximate edgelist . results in

error: (GPUassert) an illegal memory access was encountered (error 77). ../../src/RPGPUForceAtlas2.cu:232

Possibly because gravitational force doesn't overcome repulsive force that displaces small disconnected components away from the largest component, perhaps beyond the bounds of the layout frame.