Move velocity and position synchronization to appropriate place in PIC loop

[roelof-groenewald]

This is a follow-up issue from #1751, to fix the comment at https://github.com/ECP-WarpX/WarpX/blob/b858e36c24dab324f4fd951b62540ec76843ea76/Source/Evolve/WarpXEvolve.cpp#L248

See also #1751 (comment).