BUG: add missing elementsize alignment check for simd reductions

e.g. doubles are only aligned to 4 bytes on i386 so one cannot peel them
to 16 byte alignment.
Closes gh-4853
BUG: Use `np.where` in np.triu/np.tril, fixes #4859

Replaces the current method to zero items, from multiplication to
using `np.where`.
DOC: fix a typo and use TeX for plot labels
Merge pull request #4872 from juliantaylor/backports-1.9

Backports stuff from master to 1.9