Introduce attributes `snet` and `dnet` to group results by network segment

[els0r]

When there are lots of results, it might be interesting to group not by individual (sip, dip) pairs, but rather which networks communicate with each other.

DoD:

• enable this aggregation (instead of printing sip, dip, use dnet and snet)

Requires:

• drilling into the Attribute nodes
• make sure that snet and dnet can use different masks

Ideas:

• snet/24 and dnet/29 in the attributes section of a query