Allow optional first/second in relationship elements#587

JAB1305 wants to merge 2 commits into

eclipse-basyx:developeclipse-basyx/basyx-python-sdk:developfrom

rwth-iat:fix/optional-rel-element-first-second-refsrwth-iat/basyx-python-sdk:fix/optional-rel-element-first-second-refsCopy head branch name to clipboard