Feature/minor improvs log sum exp

#### Summary:
A minor improvements for log_sum_exp and an additional unit-test for log1p_exp

#### Intended Effect:

Increase efficiency of log_sum_exp function.

#### How to Verify:

Existing unit test should pass.

#### Side Effects:

None, log1p_exp(.) is at least as precise as log1p(exp(.))

#### Documentation:

Doesn't apply.

#### Reviewer Suggestions: