Tutorial 3: missing data #100

I checked some random and they are here:
https://github.com/libigl/libigl-tutorial-data

Which one is missing?

I was expecting the data to be here https://github.com/libigl/libigl-python-bindings/tree/master/tutorial/data which is basically empty. Maybe it would be enough to leave a README.md there on where to find the data?

The cmake will download the tutorial data, but this is an admittedly weird way to get it. Right now the data is downloaded optionally during build. If someone installed with pip, e.g., and just wants to try out the tutorial then it's awkward to get the data. A couple ideas:

• improve the cmake download scripts so that they can be called without building the bindings,
• download the tutorial data via python (e.g., download_data_for_tutorial.py), or
• move the tutorial to a different repo

Thanks for taking care of this!