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.
`currentStep +1` is not allowed and result in a BUG (thx @BeyondEspresso )
The reason is that the background field use `MovingWindow` offsets, but if the step `currentStep+1` the offsets and gpu positions are different.