Introduce `nifty6.mpi` namespace
As proposed by @reimar in #278 (closed), MPI parallelization can be done through specialized operators like the MetricGaussianKL_MPI.
The current state of the do_cleanup branch does not import them by default (as that would fail without mpi4py installed) and provides no easy way of importing them, making it necessary to write their full file path to import them: from nifty6.minimization.metric_gaussian_kl_mpi import MetricGaussianKL_MPI
This patch proposes to introduce a nifty6.mpi namespace, from which all MPI-related components can be directly imported like from nifty6.mpi import MetricGaussianKL_MPI.