Structural state should be a stadium shape in SBGN-like vizualization
This is how it looks now: ![image](/uploads/1e902cac8a25fefdecbdf79560e26ccf/image.png) SBGN requires: ![image](/uploads/5422da3eeaecddea8bc6286090e0b8dc/image.png) requested by `@amazein`
issue