added NaN test for inv_square
added NaN test for inv_square
added NaN and derivative doc to manual for inv_square
Merge branch 'develop' into feature/test-nan-inv-squ
Merge branch 'develop' into feature/test-nan-inv-squ
Merge pull request #977 from stan-dev/feature/test-nan-inv-squ

Feature/test nan inv squ