Bugfix/issue 900 check pos definite nan

#### Summary:
Fix check_pos_definite (issue #900).

#### Intended Effect:
Make sure check_pos_definite fails when NaN is passed in if NaN is in lower triangular of matrix. Before, if any nans are present in the matrix, the check will still pass instead of throwing.

#### How to Verify:
Run test/unit/math/error_handling/matrix/check_pos_definite

#### Side Effects:
None.

#### Documentation:
Added doc in the code.

#### Reviewer Suggestions:
Anyone.