Feature/issue 891 check pos semidefinite nan

#### Summary:
Fix check_pos_semidefinite.

#### Intended Effect:
Make sure check_pos_definite fails when NaN is passed in. 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_semidefinite

#### Side Effects:
None.

#### Documentation:
Added doc in the code.

#### Reviewer Suggestions:
Anyone.