Updates to demos to avoid segfaults

[jorgensd]

[jorgensd]

I'm not crazy about adding more syntax to a (simple) demo. Is this just for running pyvista from Docker?

These are so that we can run the codes through docker or any system that doesn’t have a frame buffer. We cannot test these otherwise. The code isn’t massively complicated by having a single if statement, and also instructs people on how to save figures.