switched to no namespace version of sqrt and exp
added NaN test for agrad/fwd/erf
updated NaN test for erf
added NaN and derivative doc for erf to manual
updated fwd/matrix tests
Merge branch 'develop' into feature/test-nan-erf

Conflicts:
	src/docs/stan-reference/appendices.tex
Merge branch 'develop' into feature/test-nan-erf

Conflicts:
	doc/stan-reference-2.4.0.pdf
	src/docs/stan-reference/appendices.tex
Merge pull request #960 from stan-dev/feature/test-nan-erf

Feature/test nan erf