It's Valentine's week, the perfect time for a visit from Miss Abigail. She is sweeping into town today and encamping in Vogel Hall at the Marcus Center. You don't know the big A? That is no shame. The central character in "Miss Abigail's Guide to Dating, Mating and Marriage," she is still an emerging star. But if you have been going to theater in Milwaukee at any time since the early `80s, you know the actress who is playing Abigail on a national tour.