Unbreak MetricGaussianKL_MPI in nifty6

In commit eda70a19, the internal config handling scheme was removed
without ensuring that MetricGaussianKL_MPI is imported. This lead to a
state in which the MPI version of the KL is never imported. Fix this by
always importing MetricGaussianKL_MPI in __init__.py.
9 jobs for nifty6_unbreak_MPI_KL in 12 minutes and 35 seconds (queued for 1 second)