Feature/test remaining functions nan

#### Summary:
This pull request includes NaN tests (including autodiff variables) and doc (with derivatives) for
- binary_log_loss
- if_else
- is_uninitalized
- int_step
- log_falling_factorial
- log_rising_factorial
- lbeta
- logit
- comparison operators
- binary operators
- modified_bessel_first_kind
- modified_bessel_second_kind
- unary operators
- multiply_log
- pow
- trunc
- round
- sign
- step
- tan
- tgamma
- owens_t
- trigamma
- binomial_coefficient_log

NOTE: this pull request should be merged after all the other NaN related pull requests. 

#### How to Verify:
Run unit tests.

### Side Effects: 
None.

#### Documentation:
Added doc in code (where applicable) and in manual.

#### Reviewer Suggestions:
Anyone.