Fix build python bindings

[jdumas]

I know we have new python bindings in the work, but right now the ones we have are not usable anymore.

• If compiled with LIBIGL_USE_STATIC_LIBRARY=ON, there will be missing symbols at runtime.
• I also fixed the path of the tutorial data in shared.py.
• Added an error message if someone tries to run cmake from the python/ folder without the correct options.
• Once this is merged, I'll put a big warning on the website that people should compile with LIBIGL_USE_STATIC_LIBRARY=ON if they want to use the python bindings.
• Also fixed a cmake issue with the line target_link_libraries(pyigl PUBLIC igl::core), that meant properties of igl::core were not correctly propagated before (including -fPIC).
• Since most python tutorials are marked as executable, I've also added a magic comment at the beginning so they can be run directly ./102_DrawMesh.py

Testing and integrating the new python bindings will take some time (a couple of weeks at least), so this needs to patched in the meantime.

Check all that apply (change to [x])

• All changes meet libigl style-guidelines.
• This is a minor change.

[jdumas]

Yes, it builds now! I've had to update a couple of third-party libraries (tetgen, triangle, stb_image, glad and comiso), but I've removed the hack around the -fPIC flag (it is now properly propagated without messing with the CXX_FLAGS globally).

[fwilliams]

If I remember correctly, the old bindings work with python 2.7. Maybe change the shebangs to python instead of python3 so we don't break people running in an environment with 2.7.

[jdumas]

Blergh, who uses python 2 anymore nowadays ...