Simplify python build

[timkpaine]

This PR seeks to replace the CMake build system with vanilla distutils for python.

Benefits

• Much more portable than before
• Vendored the header-only dependencies so no outside fetches at build time
  • boost no longer required as a system dependency
  • cmake no longer required as a system dependency
• Should support conda on all platforms "out of the box"

Negatives

• Slower builds potentially (using a numpy hack to have parallel builds)
• No modification tracking (must recompile all files every time)
  • Can work around this for our day-to-day development with ccache
• tbb required as a system dependency

[texodus]

Thanks for the PR! I think this patch has been overtaken by #1207 so I will close for now - feel free to re-open if I've missed something or this is ready for review.