Fix third party dependency installations

[nathanhhughes]

@jingnanshi This bundles a few loosely related changes:

• Switches to FetchContent instead of the old manual method for downloading source dependencies
• Switches PMC to be built and linked statically to avoid needed to install the library, and doesn't install spectra or tinyply (because they're header only and privately included)
• Updates the github actions file to use newer versions of ubuntu and adds a quick python CI test
• Removes the need for any of the RPATH shenanigans that was going on previously by statically linking registration into the python bindings

The changes (to the python bindings) mean that the python documentation is a little out of date and the conda recipes are broken, happy to go fix it at some point if you care about that

@GoldenZephyr this fixes the issue you were seeing if you want to point to this instead of the temporary fix you had

[jingnanshi]

@nathanhhughes looks great!

[ClementLeBihan]

As you removed setup.py, can you update the README ? We can't run pip install . in build/python folder.