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

Full Changelog: v5.20.5...v5.21.0