Convert the 3D printing lecture to standard format

[wenzeslaus]

The 3D printing lecture is in a custom format. It should be an assignment or assignment-like lecture with the formatting (and support files) used for everything else.

Directory with the lecture:
lectures/3d_printing

Main file:
lectures/3d_printing/index.html

Assignment document which was used as a lecture:
assignments/python_intro.html