added NaN test for log_sum_exp
added NaN test for log_sum_exp
added doc for log_sum_exp
Merge branch 'develop' into feature/test-nan-log-sum-exp
Merge branch 'develop' into feature/test-nan-log-sum-exp
Merge pull request #986 from stan-dev/feature/test-nan-log-sum-exp

Feature/test nan log sum exp