drop the 'weight' of an edge in snafun::plot_network_slices

[SNAnalyst]

when a graph has a weight attribute, snafun::plot_network_slices uses it. But this can create warnings:

In get.edge.value.active(net, sub(".active", "", attr),  ... :
  Multiple attribute values matched query spell for attribute interaction_type.active on some edges. Only earliest value used

Either:

• do not use a weight by default
• add an argument specifying if a weight is to be used and which one

[SNAnalyst]

as an example:

data("McFarland_cls33_10_16_96", package = "networkDynamic")
classroom <- cls33_10_16_96
snafun::plot_network_slices(classroom, number = 9, 
                    start = 0, end = 45,
                    digits = 3)