GNU R additional layout algorithms for network visualizations

 Several new layout algorithms to visualize networks are provided which
 are not part of 'igraph'. Most are based on the concept of stress
 majorization by Gansner et al. (2004) <doi:10.1007/978-3-540-31843-
 9_25>. Some more specific algorithms allow one to emphasize hidden group
 structures in networks or focus on specific nodes.