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

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

Conflicts:
	doc/stan-reference-2.4.0.pdf
	src/docs/stan-reference/appendices.tex
Merge pull request #952 from stan-dev/feature/test-nan-atanh

Feature/test nan atanh