Feature/issue 891 check corr matrix nan

#### Summary:
Fix check_corr_matrix so it throws if any elements of the matrix is nan. This should be merged after #905 and #901.

#### Intended Effect:
Make sure check_corr_matrix fails if elements are nan.
#### How to Verify:
Run test/unit/math/error_handling/matrix/check_symmetric

#### Side Effects:
None.

#### Documentation:
Added doc in the code.

#### Reviewer Suggestions:
Anyone.