Currently, a strand either has a DNA sequence assigned or not. This makes it awkward to implement an issue like UC-Davis-molecular-computing/scadnano#425, assigning a sequence to just one domain/loopout, where ? symbols must be assigned to domains/loopouts on the remainder of the strand.

Also, it is awkward to do calculation, since in the immutable built values Dart stores, a Strand has a reference to its Domains/Loopouts, but not the other way around. So if sequence were stored in domains, it would still be easy to calculate the DNA sequence for the whole Strand. But currently it is awkward to access the sequence of a Domain if there is no reference to the parent Strand.

This issue proposes storing the DNA sequence in the Domains/Loopouts, not only in the Dart objects, but in JSON as well.

See also UC-Davis-molecular-computing/scadnano#492.