Release 5.20.4

This is mostly a bug fix release enhancing stability and improving
performance when validating models. Main points:

• support compilation when using libxml 2.12.0 and later
• improved performance when testing for assignment cycles
• cmake build system honors the BUILD_SHARED_LIB flag if defined

What's Changed

• FIX: support compilation with libxml2 2.12.0 by @mtasaka in #358
• • ensure expat can be found when crosscompiling by @fbergmann in #368
• Replace deprecated unittest.makeSuite() by @penguinpee in #370
• Disable csharp build on nightly macOS runs by @fbergmann in #371
• add printf style formatting to Rf_error calls by @spotaws in #375
• Assignment cycle optimization by @fbergmann in #377
• Allow FBC objects to be used in both L3v1 and L3v2 core documents. by @luciansmith in #362
• Change model failures to slightly more likely mistake. by @luciansmith in #373
• Enable CMake user option BUILD_SHARED_LIB by @Waschina in #366
• Remove all checks for SBML core version when adding package elements. by @luciansmith in #380
• Prepare 5.20.4 release by @fbergmann in #381

New Contributors

• @mtasaka made their first contribution in #358
• @penguinpee made their first contribution in #370
• @spotaws made their first contribution in #375
• @Waschina made their first contribution in #366

Full Changelog: v5.20.2...v5.20.4