added NaN test for bessel_second_kind
added NaN tests for bessel_second_kind
added NaN and derivative doc for bessel_second_kind to manual
updated fwd/matrix tests
Merge branch 'develop' into feature/test-nan-bessel-secon-kind

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

Conflicts:
	doc/stan-reference-2.4.0.pdf
	src/docs/stan-reference/appendices.tex
Merge pull request #954 from stan-dev/feature/test-nan-bessel-secon-kind

Feature/test nan bessel secon kind