Vectorize FieldMatrix

Vectorize enough of FieldMatrix to make dune-geometry!13 work.

• [DONE] Fix buildsystem.
• [DONE] Ensure no performance regression.
• [DONE] Investigate anti-speedup in vectorized 1×1 solve(). (The effect disappeared in later benchmarks)
• [DONE] Merge with current master.
• [DONE] Wait for comments until at least 2016-11-17.