added NaN test for bessel_first_kind
added NaN tests for agrad/bessel_first_kind
added NaN and derivative doc for bessel_first_kind in manual
updated fwd/matrix tests
Merge branch 'develop' into feature/test-nan-bessel-first-kind

Conflicts:
	src/docs/stan-reference/appendices.tex
Merge pull request #953 from stan-dev/feature/test-nan-bessel-first-kind

Feature/test nan bessel first kind