GPU `outer_particle_loop` implementation

Some minor changes will need to be made to the `track!` interface in the `Beamlines` extension to support GPU tracking with `outer_particle_loop=true`. for example, passing the entire bunch to `outer_track!` as is done currently won't work on the GPU because it isn't `isbits`.

We will also need to think about how we can handle ramping - modifying either the `Bunch` `Brho_ref` or the `Beamline` `Brho_ref` or both (??)