RFC: Include example of vector waves (such as electromagnetism or elasticity)

Currently the machinery should work for acoustics, which is a scalar equation. But we have not yet tested for any physics with a FieldDim > 1. Without a test case it is hard to see what to do. The outer types and structure should be fine, but I expect the inner machinery will break. We also need to be careful that when we generalise the machinery fully that we don't make it slower.