Mixing matrix for edge attribute

[mbojan]

In the context of data with "weights" and/or ERGMs for valued ties it would be nice to be able do use mixingmatrix() to get some summaries of the tie values instead of tie counts. For example:

• optional edge_attr argument expecting the name of edge attribute to be summarised, defaults to NULL, i.e. count ties.
• optional argument fun expecting a function aggregating the tie values, defaults to sum

leading to expressions such as:

mixingmatrix(net, attrname = "vertex attribute", edge_attr = "weight")
mixingmatrix(net, attrname = "vertex attribute", edge_attr = "weight", fun = prod)

Thoughts? Opinions? @krivit @CarterButts ?

[martinamorris]

I like.

[krivit]

Sounds good.