In particular:

• show how to use chained methods for creating Strands (but also original way with explicit Domains)
• circular strands mean we need to be less careful with the order in which crossovers and nicks are added.
• implement write_idt_plate_excel_file uses reasonable defaults even when some strands have no IDT field set #111 first so that we don't need exporting an IDT plate file to have the side-effect of assigning IDT fields to strands (alternately, be more explicit by not calling Strand.set_default_idt, and show to how set IDT fields directly)