Vectorize FieldMatrix

Vectorize enough of `FieldMatrix` to make core/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.

See merge request !121