Compound graphs should have compound=true property

[cmungall]

As noted in https://stackoverflow.com/questions/2012036/graphviz-how-to-connect-subgraphs

It's necessary for compound graphs to have a compound=true property if clusters are to be connected.

I would have thought this sufficient:

var digraph = new Graph({compound:true});

However, it's actually necessary to do this:

digraph.setGraph({compound:true})

Which is a little obscure.

Would it be possible to set this automatically if the graph is compound?