Show bandwidth for subnetworks

[danyill]

IEC 61850 System Configurator shows bandwidth:

(It doesn't seem to show it correctly... but oh well it's hard to have everything).

Once we've figured out how to (in the publisher plugin) calculate packet size for SV and GOOSE traffic we should be able to display bandwidth as well which would be a nice feature. We could even provide warnings if we went above the SubNetwork bandwidth level.

(see OpenEnergyTools/oscd-publisher#23)