Correctly obtain the unique relation sources

As the 'relation' edge-attribute potentially holds a list of relation
sources, it needs to be unlisted first before it can be disambiguated.

This works towards #251.

Signed-off-by: Maximilian Löffler <s8maloef@stud.uni-saarland.de>

util-networks.R

@@ -1788,7 +1788,7 @@ delete.authors.without.specific.edges = function(network, specific.edge.types =
 #'         empty relation, i.e. \code{character(0)}
 get.data.sources.from.relations = function(network) {
     ## get all relations in the network
-    data.sources = unique(igraph::E(network)$relation)
+    data.sources = unique(unlist(igraph::E(network)$relation))
 
     ## map them to data sources respectively using the defined translation constant
     data.sources = sapply(data.sources, function(relation) {