Add print method for network matrices

[JBGruber]

It would be nice to have a print method for dna_network_* and dna_eventlist similar to print.dna_dataframe.

I think this should include the network type and information about the qualifier aggregation.

[leifeld]

Done for dna_network_onemode and dna_network_twomode objects. Need to do this for event lists, too, but they don't have a distinct class label yet.