Segmentation fault due to material misspelling

[jmllorens]

trafficstars

I have noticed that using the python interface, the script crashes returning a segmentation fault if the materials defining the layers do not have correspondence with any defined material.

It would be nice to introduce a checking at some point, because it is quite puzzling.

[victorliu]

The entire Python interface is currently experimental. Thanks for reporting this issue. I'll work on making the Python side of things more robust soon.