Pajek writer outputs NaN values

[szhorvat]

Describe the bug

The Pajek writes outputs NaN attribute values, but the Pajek format does not actually support these.

igraph's Pajek reader cannot read NaN values either.

This makes it impossible to test round-tripping through the Pajek format in the write_all fuzz targets.

To reproduce

write_graph(make_empty_graph() + vertex(x=NaN, y=NaN), "badpaj.net", "pajek")

Output:

*Vertices 1
1 "1" NaN NaN
*Arcs

Version information
Current master

[szhorvat]

What would be a good solution here? Perhaps skip standard Pajek attributes that contain NaN and issue a warning? igraph_vector_is_any_nan() should make it easy enough.

@krlmlr Is it easy to quickly replace all NA and NaN by a zero in a vector in R? I'd like to understand if it'd be easy for an R user to deal with this change.

[szhorvat]

I wouldn't fix this before 1.0 since @ntamas already modified the Pajek reader/writer on develop, and it is not a high-impact bug.

[szhorvat]

I suggest replacing any NaN with Pajek's defaults for that specific attribute when writing the graph.