fbpic/fbpic: 0.20.0

This release introduces several improvements and bug fixes. Better handling of multiple GPUs on clusters: The attribution of individual GPUs to the different MPI ranks is more robust, and avoids oversubscribing certain GPUs while leaving others idle (see #523). In addition, GPU oversubscription is now automatically detected, and a corresponding error message is printed. (see #524) Out-of-memory error are handled more properly, and will abort multi-GPU simulations whenever they are encountered by one of the GPUs. (see #521) The hostname is now printed even for single-rank simulation. (see #495) Kernel launch parameters have been fine-tuned for A100 architecture. (see #525) Bug fixes: The external fields used to be applied to all species, instead of the species specified by the user. This is now fixed. (see #498) The installation instructions were updated. (see #515) New features: Added a Mirror class to block the propagation of the laser e.g. in multi-stage simuations. (see #507