Additions to crossed network types

[marcelwa]

This PR adds to the functionality of crossed network types. In particular, it

• adds a new function merge_into_crossing to crossed_klut_network that merges two buffer nodes into a crossing cell
• adds new traits is_crossed_network_type, has_create_crossing, has_insert_crossing, has_merge_into_crossing
• updates the DOT drawers to support buffer nodes and crossing cells
• updates the simulation interface to support crossing cells
• fixes (?) a potential reference-counting bug in crossed_klut_network