When exporting DNA sequences, allow some reasonable defaults for ordering the strands, including row-major (i.e., lexicographically compared helix, then offset) column-major (lexicographically compared offset, then helix) of:

• 5' ends
• 3' ends
• minimum of 5' or 3' end
• start offset of top-left domain (i.e., domain that is on the minimum-index helix, which is leftmost among all such domains)

See also UC-Davis-molecular-computing/scadnano-python-package#147