Add a Domain decomposition Matrix format

[fritzgoebel]

This PR adds a new distributed matrix format useful for domain decomposition preconditioners such as BDDC. It is in preparation of eventually adding BDDC into Ginkgo.

Instead of assembling the global matrix, each rank stores its local contribution to the global matrix in a globally non-assembled state, where degrees of freedom on the subdomain interfaces are "shared" between multiple ranks. The global matrix application uses a restriction matrix R that maps into an enriched space where the shared degrees of freedom have the multiplicity of ranks sharing in them. In this space, the local contributions can be applied independently of each other before applying R^T to sum up the local parts of the result vector.

This PR relies on:

• Additive read distributed #1650
• Add local to global index mapping #1707