changed Substrand to Domain. This is a breaking change and requires naming some types, methods, and keywrod arguments.
0.3.0
removed DNAOrigamiDesign. Now there is only DNADesign. To make it an origami, called set_scaffold() on one strand, or set is_scaffold=True in the Strand constructor.
0.2.0
added ability to make helices non-consecutive. file format saved from this version on will be non-backward compatible, since helices can have an optional `idx` field not recognized by earlier versions.