option to display some domains with dashed or squiggly lines

[dave-doty]

See UC-Davis-molecular-computing/scadnano#950.

This won't affect how the Python package works, but there should be a field in the Domain and Extension classes to control this display option for the web interface.

Create an Enum type DomainDisplayType with these options:

• line (default)
• dashed
• zigzag

And add a new field display of type DomainDisplayType to both the Domain and Extension classes.