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 sbmlteam/libsbml#441
• Better testing of reaction to raterule by @skeating in sbmlteam/libsbml#442
• • new groups / curves need to be added to the list of drawables by @fbergmann in sbmlteam/libsbml#445
• Sk/test rate rule converter rigorously by @skeating in sbmlteam/libsbml#450
• Added missing include in html2md by @vincent-noel in sbmlteam/libsbml#440
• Test whether big/little endian using newer constructs. by @luciansmith in sbmlteam/libsbml#437
• #453: check for NULL, ensure boundary condition is reset by @fbergmann in sbmlteam/libsbml#454
• Add functions to translate gene product associations to and from ASTNodes by @luciansmith in sbmlteam/libsbml#449
• #455: ensure we dont crash on no children by @fbergmann in sbmlteam/libsbml#456
• Sk/log converter errors by @skeating in sbmlteam/libsbml#457
• Sort multiple compartments by @skeating in sbmlteam/libsbml#464
• Adds all the derivative values by @skeating in sbmlteam/libsbml#463
• Bump version in preparation of new release by @fbergmann in sbmlteam/libsbml#466
• Allow to bypass suitability tests in raterule connverter by @fbergmann in sbmlteam/libsbml#465
• current release updated to list 5.21 by @fbergmann in sbmlteam/libsbml#467
• #438: mark writeMathMLWithNamespaceToString as newobject by @fbergmann in sbmlteam/libsbml#468

Full Changelog: sbmlteam/libsbml@v5.20.5...v5.21.0