Create a clean distributed-ginkgo branch

[MarcelKoch]

To ease the development of distributed functionality for ginkgo, there should exist a branch distributed-ginkgo that contains the current and agreed-upon status.

For this, I want to incrementally add the functionality that is already in the branch mpi-base-dist-mat into the new branch. Currently, I think this will require PR for the following functionality:

• MPI bindings Add a MPI layer, some bindings, CMake setup and MPI test framework #908 (1)
• a partition of global indices Add a class for partitions of intervals #909 (2)
• distributed vectors (3) Distributed vector #961
• distributed matrices (4) Adds support for MPI distributed matrix #971
• distributed solvers (5) Adds distributed support for several solvers #976
• enable logging of MPI communications (6)
• add distributed domain decomposition predonditioner (7)
• figure out mutable access to local vectors and matrices (8) Reusable read #1159

There are the following dependencies between these tasks:

• (5) → (4) & (3)
• (4) & (3) → (2) & (1)
• (6) → (1)
• (7) → (4) & (3)
• (8) → (4)

At the moment, there are no dependencies between (1) and (2).