Exchange a package that is marked as deprecated.
Seen in CI as warning in the apt install step with CUDA 10.1.243.

To Do:

• double check this works with CUDA 9.2 (current docker image; check minimal version constraint for Nvidia NGC) or...
• update Docker image to newer CUDA

The WarpX documentation (esp. the theory part) uses a mix of SI units, Gaussian units, natural units, in different sections/equations.

We should standardize the whole documentation so that SI units are used everywhere.