Picongpu: Laser Antenna

[ax3l]

I think it would be very beneficial to add a new antenna.param input file. This file could be used to efficiently initialize laser pulses via currents.

@hightower8083 already uses background fields in PoGit for this and I think we should make this a core input option.

Re: #619
cc @BeyondEspresso @PrometheusPi @steindev @n01r

[PrometheusPi]

@ax3l great proposal (I definitely support this.) We need someone to work on this.

[sbastrakov]

We just discussed it with @BeyondEspresso . I will make a prototype for it to play around. Hopefully next week for something working with reduced functionality.