Add a Boolean field Strand.circular and draw a crossover from last substrand to first. Any equivalent cyclic permutation of the substrands should display in the same way.

Even if one doesn't wish the final design to have circular strands, this will help to avoid the problem that sometimes an intermediate design temporarily creates a circular Strand, even though subsequent edits make it linear again. Currently such designs are simply not allowed.

See UC-Davis-molecular-computing/scadnano#5.