Navier-Stokes problem for distributed memory AMGCL benchmarks

The system matrix and the right hand side used in the distributed Navier-Stokes benchmark at https://github.com/ddemidov/amgcl_benchmarks.

This is an incompressible Navier-Stokes problem discretized on a non uniform 3D mesh with a finite element method, using an equal-order tetrahedral Finite Elements, stabilized employing an ASGS-type (algebraic subgrid-scale) approach. This results in a discretized linear system of equations with a 4x4 block structure. The overall system matrix for the problem was assembled in the [Kratos](http://www.cimne.com/kratos/) multi-physics package developed in CIMNE, Barcelona. The matrix contains 4'773'588 unknowns and 281'089'456 nonzeros.