Series: Split Serial and MPI-Parallel

[ax3l]

We should provide separate includes for serial and MPI-parallel openPMD, e.g.

#include <openPMD/openPMD.hpp>

and

#include <openPMD/MPI.hpp>

as well as two separate, non-compile-flag modified Series objects in those (potentially in two libs), e.g. constructed in a make_Series( ... ); helper (and disallow all constructors, which also solves handler/copy problems).

This will enable workflows with downstream usage where a serial program wants to use an MPI-parallel openPMD-api library without having to include or link superfluous MPI libraries, too.