Miscellaneous fixes to code and documentation.

What's Changed

• Resolve cyclic dependency of Body.vsopModel by @ebraminio in #242
• Prevent use of gcc "fast math" optimization in the C version, because it breaks things! See #245.
• Various minor fixes to Python documentation.
• Bump shell-quote from 1.7.2 to 1.7.3 in /generate by @dependabot in #232

Full Changelog: v2.1.2...v2.1.3