add degree as a technical attribute

[paulgirard]

As a user not seeing degree as node attributes is weird. Using degree is possible but users have to first compute the metrics Degree.

One possible way to simplify this would be to add degree and weighted degree in the list of technical attributes see #110
It would require to make sure the attribute is always updated when structure changes but it would allow filters/appearance to react dynamically when using it.

Note: this is Desktop Gephi current behaviour

To be discussed