Evidence — Expert testimony — District court, acting pursuant to Federal Rule of Evidence 702 and Daubert v. Merrell Dow Pharmaceuticals, Inc. did not err in excluding opinions of plaintiff’s proffered expert witness as unsound, where expert failed to meaningfully address dose-response with regard to plaintiff, other potential causes of plaintiff’s medical conditions or the background risk of those conditions and this fundamental methodological failure undermined the soundness of his causation opinions