Particles Container: Polymorphic Arena & Runtime

[ax3l]

As discussed, in order to simplify Python variants and code coupling, we want to make two changes to the WarpX particle containers:

• use the PolymorphicArena to have the CPU/GPU/pinned allocation decided at runtime (as we do for MultiFabs)
• make every component (including/excluding positions, but ideally including them) a runtime attribute

@atmyers volunteered to lead this effort in CAMPA.