Calculate bandwidth for SMV and GSE control blocks

[danyill]

It would be nice to be able to calculate bandwidth for SMV and GSE control blocks.

• This would be useful to limit traffic when building a dataset (Show packet size for GOOSE and SMV datasets oscd-publisher#23)

• This would be useful in showing information in the Communication editor (Show bandwidth for subnetworks scl-communication#5).

This could be useful in other plugins to do performance calculations in future.

[JakobVogelsang]

I will add this fairly soon. This is a function that needs to exist when you want to encode a GOOSE packet and can be moved to JavaScript/TypeScript as well.