Add BG Fields in Plugins

Part of @BeyondEspresso changes for the TWTS implementation in the original pull #597.

The bg fields are now still available during the plugin/output phase for easier debugging.

A follow up could write the background fields to an independent output field.
yes, but it should be added *before* `runOneStep` for initializations, e.g., for GPUs that got created on top of the globalDomain during a slide.