Add support for SONIA file format

[Gábor Csárdi]

Quite simple format, so it is easy:

http://sourceforge.net/apps/mediawiki/sonia/index.php?title=Son_format
http://www.stanford.edu/group/sonia/exampleFiles/

Here is a quick and dirty R function for the sonia -> igraph conversion:

sonia2igraph <- function(filename, directed=TRUE) {
  lines <- readLines(filename)
  lines <- grep("^//", lines, value=TRUE, invert=TRUE)
  fromid <- grep("^FromId", lines)[1]
  nodes <- lines[1:(fromid-1)]
  edges <- lines[fromid:length(lines)]

  nodes <- strsplit(split="\t", nodes)
  nodesnames <- nodes[[1]]
  nodes <- data.frame(do.call(rbind, nodes[-1]))
  colnames(nodes) <- nodesnames
  colnames(nodes)[1] <- "name"

  edges <- strsplit(split="\t", edges)
  edgesnames <- edges[[1]]
  edges <- data.frame(do.call(rbind, edges[-1]))
  colnames(edges) <- edgesnames

  graph.data.frame(edges, directed=directed, vertices=nodes)
}

[Gábor Csárdi (gabor.csardi)]

The development of igraph has moved to github, so please do not comment on this bug here. You are of course welcome to comment on github, here:
https://github.com/igraph/igraph/issues/340