Galvanic cell diagram drawing fails if the font "arial.ttf" does not exist

[simeneilevstjonn]

The galvanic cell drawer assumes there exists a font called "arial.ttf". This is the case on Windows, but not on Linux or Mac, leading to an OSError when attempting to draw such a diagram.

There could perhaps be different fonts based on platform, or might be possible to use the load_default() method.