Feature/simd for multi rhs

Update the solvers to solve for multiple rhs vectors simultaniously. This makes use of SIMD abstractions in dune-common (see dune-common!16 (merged)) to use vectors of SIMD vectors as FieldType of the right-hand-side. By this we can make immediate use of SIMD units in modern processors. Using SSE for example we get a speedup of 2 basically for free. This extension is useful whenever the same operator has to be solved for many rhs vectors, e.g. in the case of MS-FEM.

AMG Ist working as long as the coarse grid solver is an iterative solver. I therefore added an additional preprocessor macro to disable the direct coarse solver explicitly.

What is missing currently is multi-rhs support for the direct solvers. I'm preparing a separate merge request for this feature, which will add multi-rhs direct-coarse-grid support for the AMG.

See also: dune-common!81 (merged), dune-geometry!13 (closed).