sbmlteam/python-libsbml: Release 5.21.0

This release further improves the ODE to Reaction converter, improves the ASTNode::derivative function to support all allowed math constructs, and resolves a number of build and stability issues. What's Changed Update html2md. by @luciansmith in https://github.com/sbmlteam/libsbml/pull/441 Better testing of reaction to raterule by @skeating in https://github.com/sbmlteam/libsbml/pull/442 new groups / curves need to be added to the list of drawables by @fbergmann in https://github.com/sbmlteam/libsbml/pull/445 Sk/test rate rule converter rigorously by @skeating in https://github.com/sbmlteam/libsbml/pull/450 Added missing include in html2md by @vincent-noel in https://github.com/sbmlteam/libsbml/pull/440 Test whether big/little endian using newer constructs. by @luciansmith in https://github.com/sbmlteam/libsbml/pull/437 #453: check for NULL, ensure boundary condition is reset by @fbergmann in https://github.com/sbmlteam/libsbml/pull/454 Add functions to translate gene product associations to and from ASTNodes by @luciansmith in https://github.com/sbmlteam/libsbml/pull/449 #455: ensure we dont crash on no children by @fbergmann in https://github.com/sbmlteam/libsbml/pull/456 Sk/log converter errors by @skeating in https://github.com/sbmlteam/libsbml/pull/457 Sort multiple compartments by @skeating in https://github.com/sbmlteam/libsbml/pull/464 Adds all the derivative values by @skeating in https://github.com/sbmlteam/libsbml/pull/463 Bump version in preparation of new release by @fbergmann in https://github.com/sbmlteam/libsbml/pull/466 Allow to bypass suitability tests in raterule connverter by @fbergmann in https://github.com/sbmlteam/libsbml/pull/465 current release updated to list 5.21 by @fbergmann in https://github.com/sbmlteam/libsbml/pull/467 #438: mark writeMathMLWithNamespaceToString as newobject by @fbergmann in https://github.com/sbmlteam/libsbml/pull/468 Full Changelog: https://github.com/sbmlteam/libsbml/compare/v5.20.5...v5.21.0