Lists as vertex/edge attributes

[mbojan]

asNetwork and asIgraph do not handle lists as vertex or edge attributes.

Need to add tests so that errors/warnings are triggered when trying to convert objects containing such attributes.

Think of ways of properly handling them: are they supported by both igraph and network?

[mbojan]

igraph does support lists as vertex/edge attributes:

g <- make_graph(~ A --+ B:C)
V(g)$x <- list(1:2, 3:4, letters[1:5])
V(g)$x

E(g)$w <- list(1:2, letters[1:4])
g