Enable vectorization of vector updates in Chebyshev preconditioner

Use DEAL_II_OPENMP_SIMD_PRAGMA annotation. To avoid a bug in gcc, use an additional
boolean in the 'if' statement that avoids checking factor1 == Number().
Re-fill comments in precondition.h
Merge pull request #252 from kronbichler/master

Enable vectorization of vector updates in Chebyshev preconditioner