Improved AVX-512 vectorization code path

The original version of the AVX-512 vectorization (DEAL_II_VECTORIZATION_LEVEL = 3) was not properly tested. When I did some tests with gcc, it turned out that I had to fix a few issues. Now it compiles fine and the generated assembler code looks good, but there is no hardware out to test it yet.
I also improved some code comments.
Merge pull request #202 from kronbichler/master

Improved AVX-512 vectorization code path. The original version of the AVX-512 vectorization (DEAL_II_VECTORIZATION_LEVEL = 3) was not properly tested. When I did some tests with gcc, it turned out that I had to fix a few issues. Now it compiles fine and the generated assembler code looks good, but there is no hardware out to test it yet.