… Strand, and added example of creating one with chained method calls.

…rands

closes #14; add support for circular Strands

…rands

updated add_half_crossover and add_nick to handle circular strands

…loopout, nicking on all three domains

…ver, since issue #111 still need to be implemented)

…. Two tests added: which come from cadnano Autostaple's output

…_inputs/cadnano_v2_export as some .sc files are not generated but directly read by the tests

…import-circular-strands

Dev export import circular strands