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

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

Conflicts:
	doc/stan-reference-2.4.0.pdf
	src/docs/stan-reference/appendices.tex
Merge pull request #951 from stan-dev/feature/test-nan-atan

Feature/test nan atan