added NaN test for lmgamma
added NaN test for lmgamma
added NaN and derivative doc in manual for lmgamma
Merge branch 'develop' into feature/test-nan-lmgamma
Merge branch 'develop' into feature/test-nan-lmgamma

Conflicts:
	src/docs/stan-reference/appendices.tex
Merge pull request #980 from stan-dev/feature/test-nan-lmgamma

Feature/test nan lmgamma