Set up ramps in `PlasmaLinac.assemble_trackables()`

[ben-c-2013]

Currently, the ramps are set up when individual Stage.track() are called, which in turn calls Stage._prepare_ramps(). As mentioned in PR #200, but a better solution may be to call Stage._prepare_ramps() in PlasmaLinac.assemble_trackables() for all stages and set up all ramps there instead.