Fixed a typo
Added maxabs and minabs functions
Added missing header
Merge branch 'expressions' of https://github.com/ValeevGroup/tiledarray into expressions

Conflicts:
	src/TiledArray/math/vector_op.h
Merge branch 'expressions' of https://github.com/ValeevGroup/tiledarray into expressions
Fixed a mysterious compilation error with clang++ on a Mac. p.s. Justus does not see this issue! Same compiler and library, only slight differences in include path