(noting that individualized damage issues could swamp the advantages coming from an initial, class- wide liability determination); accord Allison v. Citgo Petroleum Corp., 151 F.3d 402, 421-22 (5th Cir. 1998), cited with approval in Gunnells v. Healthplan Servs., Inc., 348 F.3d 417, For internal use only SDNY_GM_000