Remove plot.network()'s reliance on the internal representaion of 'network'?

[krivit]

Specifically,

network/R/plot.R

Line 804 in 769972a

use<-displayisolates|(((sapply(x$iel,length)+sapply(x$oel,length))>0))

relies on $oel and $iel solely to determine which nodes are isolates.
This means, e.g., networkLite objects has to implement its own function.

Probably the most elegant fix for this is to move the is.isolate() or maybe even degree() to network and make them generics, then have sna implement methods for whatever types it supports.

@CarterButts , thoughts?