force layout: stabilize stacked pseudoknot base pairs

[martin-raden]

observation

• "pseudoknot" base pairs are without strength and further (hidden) constraints in standard fornac layouting
• enabling "Pull Pseudoknot Basepairs" layouting sets the layout strength to the same value as normal base pairs

to check and implement

• are there hidden constraints in the force field to stabilize the rectangular form of stacked/neighbored "basepair" edges?
  • e.g. there might be diagonal edges to pull the oposite corners towards each other
  • if not: add such constraints to stabilize the layouting of stackings
• if "basepair" edges are constraint: are stacked/neighbored "pseudknot" base pairs similarly constraint?
  • if not: add such constraints for stacked "pseudoknot" edges if "Pull Pseudoknot Basepairs" is enabled