Merge branch 'feature/issue-891-check-pos-semidefinite-nan' into feature/issue-891-check-spsd-matrix-nan
added NaN test for check_spsd_matrix
added doc in code for check_spsd_matrix
Merge pull request #937 from stan-dev/feature/issue-891-check-spsd-matrix-nan

Feature/issue 891 check spsd matrix nan